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

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[viragtripathi]

/assign @XuanYang-cn