YT-27780: Support push-based shuffle in shuffle service

* Changelog entry
  Type: feature
  Component: proxy

Add push-based shuffle to the YT shuffle service.

What it is: instead of each mapper writing its own table chunks (pull-based), all mappers for a partition append to one shared journal chunk through a per-partition sequencer that orders the concurrent appends.

How to use it: pass use_push_based_shuffle=true to start_shuffle, together with a strict TTableSchema (required for push-based; it is the single source of the column name-to-id mapping). The flag is opaque to the client and rides on the signed shuffle handle. Pull-based shuffle is the default and stays byte-identical.
commit_hash:0f2409932b7092913edee03360b2be9f2495aafd

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