feat: add Google BigQuery driver plugin

[datlechin]

Summary

• New BigQuery driver plugin (Plugins/BigQueryDriverPlugin/, 9 files, ~3,500 lines) as a registry plugin
• Auth: Service Account JSON (JWT/RS256), Application Default Credentials, Impersonated Service Account, OAuth 2.0 browser flow
• Full GoogleSQL support: dataset browsing, table structure (STRUCT/ARRAY/nested types), query execution via Jobs API, data editing (INSERT/UPDATE/DELETE), dry run cost estimation
• Query cost displayed after every execution (bytes processed, billed, estimated USD) via new PluginQueryResult.statusMessage field
• Tagged query system for pagination (BigQuery has no OFFSET), ROW_NUMBER() windowing for ad-hoc SQL
• 429 rate limit retry with exponential backoff, configurable query timeout, schema cache with 5-min TTL
• 30 unit tests (QueryBuilder, StatementGenerator, TypeMapper)
• Docs page, CHANGELOG entry, status tracker

Test plan

• Build succeeds: xcodebuild -project TablePro.xcodeproj -scheme TablePro build -skipPackagePluginValidation
• All 30 BigQuery tests pass: xcodebuild test -only-testing:TableProTests/BigQuery*
• Connect with Service Account JSON key
• Connect with ADC (gcloud auth application-default login)
• Connect with OAuth (browser flow)
• Browse datasets and tables in sidebar
• Execute SELECT query, verify cost shown in status bar
• Execute INSERT/UPDATE/DELETE, verify rowsAffected + cost in success view
• Dry Run (EXPLAIN) shows bytes processed and estimated cost
• Browse table with pagination (page 2+)
• Filter and search on table data
• Disconnect and reconnect cleanly