feat: databricks tags merge behavior

[canbekley]

Resolves #1184
Resolves #1188

Description

defining databricks_tags on a lower configuration level currently completely replaces/clobbers any tag configuration defined at higher level (e.g. in dbt_project.yml). this is very unintuitive for dictionary type configurations and is usually solved with additive dictionary merging behaviour (e.g. meta, tags, grants etc.).

dbt-common implements the MergeBehavior enum that can be used to set the meta_data of the dataclass fields in DatabricksConfig, which is handled by the BaseConfig. defining MergeBehavior.Update for databricks_tags basically makes it an additive dictionary field.

Note:

• because of some test overlaps, the branch here is branched off of feat/support-for-key-only-databricks-tags from pull request: feat: support key-only databricks_tags #1339
• this implementation could probably be extended to the tblproperties configuration as well, as those suffer the same limitations at the moment. I could add it in here if you like.

Checklist

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