feat: Add CockroachDB vector database support

[viragtripathi]

 Implements comprehensive CockroachDB integration with these features:

 Core Features:
 - CockroachDB vector index support using C-SPANN algorithm (v25.2+)
 - Production connection pooling (100+ base connections, configurable)
 - Automatic retry logic for transient errors (40001, 40003)
 - Multi-region resilience with proper error handling
 - Support for L2, Cosine, and Inner Product distance metrics
 - Vector index tuning (partition size, beam size parameters)
 - Filter support (NumGE, StrEqual)

 Implementation:
 - vectordb_bench/backend/clients/cockroachdb/cockroachdb.py: Main client with retry logic
 - vectordb_bench/backend/clients/cockroachdb/config.py: Configuration classes
 - vectordb_bench/backend/clients/cockroachdb/db_retry.py: Retry decorator with exponential backoff
 - vectordb_bench/backend/clients/cockroachdb/cli.py: CLI parameter definitions
 - tests/test_cockroachdb.py: Comprehensive test suite
 - pyproject.toml: Added cockroachdb optional dependencies

 References:
 - CockroachDB Vector Indexes: https://www.cockroachlabs.com/docs/stable/vector-indexes
 - Retry Best Practices: https://cockroachlabs.com/docs/stable/advanced-client-side-transaction-retries

[viragtripathi]

/assign @XuanYang-cn

[alwayslove2013]

@viragtripathi Thank you so much for your contribution!

We noticed there are some conflicts in this pull request. Could you please resolve them? After that, we'll be happy to merge it.

[viragtripathi]

@viragtripathi Thank you so much for your contribution!

We noticed there are some conflicts in this pull request. Could you please resolve them? After that, we'll be happy to merge it.

@alwayslove2013 Thank you!
I've resolved the conflicts by rebasing on the latest main branch. The implementation is ready for review and merge. Please let me know if you need any changes!

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