[WIP] Allow Explicit Merge actions

[mi-volodin]

Resolves #949

Description

This PR extends the capability of Merge strategy configuration and allow to completely override actions block with an explicit statement.

E.g. it allows to define multiple actions with multiple conditions and tune this part at developer's convenience.

when matched and ... then update
...
when matched and ... then delete
when not matched and ... then insert
...

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.

[mi-volodin]

There are some extra actions on docs + changelog, but the code is ready for review.

@benc-db could you please trigger the tests on your side? My functional tests with snapshots traditionally doesn't work.

[benc-db]

Nice. I'll need to sleep on the name of the config, but the approach and test looks good.

[mi-volodin]

Yep, of course, I'll write a docs entry meanwhile.

[mi-volodin]

For transparency - it was decided to pause this PR activities until the principal decision would be made to introduce that functionality in that form.