Custom Transformations: Implement API configurations

[ChristopheDuong]

Tell us about the problem you're trying to solve

As part of project #2959, this ticket should track progress on the API/configurations part of the work

Describe the solution you’d like

There should be an optional list of transformation objects (at the connection level, not at the stream level) tied to the StandardSync configurations.

Additional context:

Google Doc - Tech Specs