feat: add SQL Server connection pool

[gnolong]

Summary

• add SQL Server connection configuration, validation, and TLS mapping
• add a bb8/Tiberius connection pool with session reuse protection and ConnClient wiring
• add real SQL Server integration coverage for pool limits, Tokio tasks, transactions, and dirty-session eviction

Scope

• does not construct MSSQL snapshot extractor or sinker components
• component factories return explicit not-implemented errors
• metadata, type conversion, snapshot extraction, and sink logic remain in feat: scaffold SQL Server snapshot tasks #562

Testing

• cargo fmt --all -- --check
• cargo check --workspace
• cargo test -p dt-common
• cargo test -p dt-tests mssql_to_mssql::connection_pool_tests -- --nocapture