Move use_user_folder_for_python to behavior

[benc-db]

Description

Refactors switching to user folder to be behind a behavior flag, since the expectation is that ultimately Shared will be deprecated.

Checklist

• [x] I have run this code in development and it appears to resolve the stated issue
• [x] This PR includes tests, or tests are not required/relevant for this PR
• [x] I have updated the CHANGELOG.md and added information about my change to the "dbt-databricks next" section.