add public roadmap (KIKIMR-15090)

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+# YDB Roadmap
+
+## Query Processor
+1. [ ] Support for **Snapshot Read-only** transactions mode
+1. [ ] **Better resource management** for KQP Resource Manager (share information about nodes resources, avoid OOMs)
+1. [ ] Switch to **New Engine** for OLTP queries
+1. ✅ Support **`not null` for table columns**
+1. [ ] **Aggregates and predicates push down to column-oriented tables**
+1. [ ] **Optimize data formats** for data transition between query phases
+1. [ ] **Index Rebuild**
+1. [ ] Optimize **KQP Session Actor** (optimize to reduce CPU usage)
+1. [ ] **PostgreSQL compatibility**
+    * [ ] Support PostgreSQL datatypes **serialization/deserialization** in YDB Public API
+    * [ ] PostgreSQL compatible **query execution** (TPC-C, TPC-H queries should work)
+    * [ ] Support for PostgreSQL **wire protocol**
+1. [ ] Support a single **Database connection string** instead of multiple parameters
+1. [ ] Support **constraints in query optimizer**
+1. [ ] **Query Processor 3.0** (a set of tasks to be more like traditional database in case of query execution functionality)
+    * [ ] Support for **Streaming Lookup Join** via MVCC snapshots (avoid distributed transactions, scalability is better)
+    * [ ] **Universal API call for DML, DDL, ScanQuery** (aka StreaminExecuteQuery, which allows to execute each query)
+    * [ ] Support for **secondary indexes in ScanQuery**
+    * [ ] **Transaction can see its updates** (updates made during transaction execution are not buffered in RAM anymore, but rather are written to disk and available to read by this transaction)
+1. [ ] **Query graphs caching at DataShards** (optimize CPU usage)
+1. [ ] **RPC Deadline & Cancellation propagation** (smooth timeout management)
+1. [ ] **DDL for column-oriented tables**
+2. [ ] **Select indexes automatically**
+
+## Database Core (Tablets, etc)
+1. [ ] Get **YDB topics** (aka pers queue, streams) ready for production
+1. ✅ Turn on **MVCC support** by default
+1. [ ] Enable **Snapshot read mode** by default (take and use MVCC snapshot for reads instead of running distributed transaction for reads)
+1. [ ] **Change Data Capture** (be able to get change feed of table updates)
+1. [ ] **Async Replication** between YDB databases
+1. ✅ **Background compaction for DataShards**
+1. [ ] **Compressed Backups**. Add functionality to compress backup data
+1. [ ] Process of **Extending State Storage** without cluster downtime. If a cluster grows from, say, 9 nodes to 900 State Storage configuration stays the same (9 nodes), it leads to a performance bottleneck.
+1. [ ] **Splite/Merge DataShards *BY LOAD* by default**. Most users require this feature turned on by default
+1. [ ] Support **PostgreSQL datatypes** in tablet local database
+1. [ ] **Basic histogram for DataShards** (first step towards cost based optimizations)
+1. [ ] **Transaction can see its updates** (updates made during transaction execution are not buffered in RAM anymore, but rather are written to disk and available to read by this transaction)
+1. [ ] **Data Ingestion from topic to table** (implement built-in compatibility to ingest data to YDB tables from topics)
+1. [ ] Support **snapshot read over read replicas** (consistent reads against read replicas)
+
+### Hardcore
+1. [ ] **Datashard iterator reads via MVCC**
+1. [ ] **Switch to TRope** (or don't use TString/std::string directly, provide zero-copy data passing between components)
+1. [ ] **Avoid Node Broker as SPF** (NBS must work without Node Broker under emergency conditions)
+1. [ ] **Subscriptions in SchemeBoard** (optimize interaction with SchemeBoard via subsription to updates)
+
+## BlobStorage
+1. [ ] **"One leg" storage migration without downtime** (migrate 1/3 of the cluster from one AZ to another for mirror3-dc erasure encoding)
+1. [ ] **ActorSystem 1.5** (dynamically reassign threads in different thread pools)
+1. [ ] **Publish an utility for BlobStorage management** (it's called ds_tool for now, improve it and open)
+1. [ ] **Self-heal for degrated BlobStorage groups** (automatic self-heal for groups with two broken disks, get VDisk Donors production ready)
+1. [ ] **BlobDepot** (a component for smooth blobs management between groups)
+1. [ ] **Avoid BSC (BlobStorage Controller) as SPF** (be able to run the cluster without BSC in emergency cases)
+1. [ ] **BSC manages static group** (reconfiguration of the static BlobStorage group must be done BlobStorage Controller as for any other group)
+1. [ ] **(Semi-)Hard disk space separation** (Better guarantees for disk space usage by VDisks on a single PDisk)
+1. [ ] **Reduce space amplification** (Optimize storage layer)
+1. [ ] **Storage nodes decommission** (Add ability to remove storage nodes)
+
+## Analytical Capabilities
+1. [ ] **Log Store** (log friendly column-oriented storage which allows to create 1+ million tables for logs storing)
+1. [ ] **Column-oriented Tables** (introduce a Column-oriented tables in additon to Row-orinted tables)
+1. [ ] **Tiered Storage for Column-oriented Tables** (with the ability to store the data in S3)
+
+## Federated Query
+1. [ ] **Run the first version**
+
+## Embedded UI
+1. [ ] **Support for all schema entities**
+    * [ ] **YDB Topics Support** (add support for viewing metadata of YDB topics, its data, lag, etc)
+    * [ ] **CDC Streams**
+    * [ ] **Secondary Indexes**
+    * [ ] **Read Replicas**
+    * [ ] **Column-oriented Tables**
+
+## Command Line Utility
+1. [ ] Use a **single `ydb yql`** instead of `ydb table query` or `ydb scripting`
+1. [ ] Interactive CLI
+
+## Tests and Benchmarks
+1. [ ] **Built-in load test for DataShards** in YCSB manner
+1. [ ] **`ydb worload` for topics**
+1. [ ] **Jepsen tests support**
+
+## Experiments
+1. [ ] Try **RTMR-tablet** for key-value workload