chore: include BigQuery and Databricks in all-dbs extra or rename it

[dtsong]

Problem

pyproject.toml defines an all-dbs optional dependency group that installs 11 of 13 database drivers. BigQuery and Databricks are excluded. Users installing data-diff[all-dbs] don't get all databases.

Options

1. Include BigQuery and Databricks in all-dbs
2. Rename to common-dbs and add a separate all-dbs that includes everything
3. Document the exclusion in README

Acceptance Criteria

• all-dbs extra either includes all databases or is honestly named
• README documents available extras