feat: Add YDB support to the benchmark

[zinal]

YDB is a distributed SQL database which supports vector search, including the exact and indexed (approximate) search.

This PR adds basic support for YDB to the VectorDBBench code.

Fixes #792

Notes for maintainers

Adding a YDB client requires small, opt-in extensions to shared benchmark infrastructure. The goal is to support gRPC-based clients and case-specific index DDL without changing behavior for existing databases.

All changes follow the same pattern already used for thread_safe in concurrent_runner.py: boolean capability flags on VectorDB, checked via getattr(..., False). No database-specific if db == DB.X branches remain in shared runners.

YDB uses a gRPC driver and builds vector_kmeans_tree indexes after data load. Index DDL depends on the case filter type. That creates three practical constraints:

1. Subprocess + pickle — the YDB driver/session pool cannot be pickled when ProcessPoolExecutor forks or spawns a child process.
2. Filters at init time — index shape must be known when optimize() runs, not only during search.
3. Per-case table names — same requirement as for Doris: avoid cross-case interference when running multiple benchmarks against one cluster.

These are client-specific needs, not general benchmark changes.

[sre-ci-robot]

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