Add custom constraint option

[roydobbe]

Description

Added an option to create custom constraints. A usecase could be creating TIMESERIES primary keys.

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.