chore(connector): Deprecate connectors.base

[john-bodley]

SUMMARY

I was running into CI issues in #22413 which I suspect maybe related to the fact that the base datasources/columns/metrics which might not adhere to the SQLAlchemy relationship model, i.e., BaseDatasource.columns/BaseDatasource.metrics are defined as Python lists as opposed to a SQLAlchemy.orm.relationship.

Somewhat of a Yak Shaving experience, this PR simply deprecates the abstract classes—BaseDatasource, BaseColumn, and BaseMetric—which are no longer needed given that we no longer support the native Druid connector. Note this refactor effort is far from complete—there remains endless spaghetti code which uses a datasource type mapping—but hopefully it's a step (albeit small) in the right direction.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[john-bodley]

This was never used. I'm surprised Pylint didn't recognize it, but maybe it's not well versed with the TYPE_CHECKING construct and thus it felt it was used.