YQL-19747 Introduce Union and split Static NameServices

Essentially we need the `UnionNameService` to integrate dynamic name services into the `sql/v1/complete`. This decorator queries multiple name services and merges their responses. We will use it to union static `NameServices` and the `SchemaNameService` (and the `ClusterNameService`, and so on).

It does not deal with exceptions and fails the whole request on some subtask failed. Client should prepare robust children (later `SwallowingNameService` will be added to return an empty `NameResponse` on errors to provide best-effort dynamic object loading).

`StaticNameService` was split into micro-`NameService`s :)

`NameConstraints` are extracted to provide name qualification via `Qualified` and `Unqualified` methods. This is needed because depending on a context `NameService` can return unqualified names (for example, on `PRAGMA yt.#`). As internal indexes for scanning and ranking are built on a sorted list of, then `Ranking` actually needs an ability to get fully-qualified names, so now it via the `NameConstraints`.

Also this design potentially let us to improve internal indexes by using partitioning by a namespace. Other option was to make `PragmaName` and `FunctionName` more structured via adding a separate field for a namespace, but it seems to me that it will force to do more parsing from indexes results to a `Name`. Anyway this is an internal component so it can be changed if needed.  I still doubt this decision because structured `PragmaName { Namespace, Identifier }` seems to be cleaner and there should be no noticeable overhead because of COW strings.

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Pull Request resolved: https://github.com/ytsaurus/ytsaurus/pull/1252
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