fix: Set DatabricksConnectionConfig to only use a shared_connection with U2M OAuth

[davem-bis]

Description

The DatabricksConnectionConfig was set to use a shared connection pool for all authentication types to resolve a race condition with U2M OAuth, however this has the side effect that SQL execution is sequential regardless of the concurrent_tasks config setting.

This change updates:

• The shared_connection variable to be a computed field for all ConnectionConfig classes.
• The shared_connection variable to return True only for U2M OAuth authentication methods for the DatabricksConnectionConfig class.

Partially resolves #5858.

Test Plan

• Unit testing for all 3 situations
• Tested on Python 3.12.3, SQLMesh 0.253.3, databricks-sql-connector 4.0.5 with Access Token and U2M OAuth.

Checklist

• I have run make style and fixed any issues
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test)
• My commits are signed off (git commit -s) per the DCO