Added `spark_mode` parameter to allow switching between spark and databricks connect

[MigQ2]

Description

Fixes #700

Checklist

• [X] Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• [ ] Updated the documentation to reflect the code changes
• [X] Added a description of this change in the relevant RELEASE.md file
• [ ] Added tests to cover my changes

[noklam]

Is this ready to be reviewed? I see there are still a few failing tests.