Search Kit: Support robust joins in UI

[colemanw]

Overview

Vastly improves support for joins in the Search Kit UI.

Before

Only a few joins supported, others appeared to work but would fail due to ambiguities in the multiple ways 2 entities can be joined.

After

This uses better join metadata to give explicit conditions for each join, removing the ambiguity when more than one type of join is possible between the same 2 entities.
It also adds support for joins via EntityBridge APIs.

[civibot]

(Standard links)

• If this is your first pull-request for CiviCRM, please browse CONTRIBUTING.md for information about the development and testing processes.
• If you are reviewing this pull-request, you may wish to consult the test sites and the Review Standards (long template, short template).

[eileenmcnaughton]

@colemanw given we are still stabilising search kit and this only touches files in there my preference is that you merge this as soon as you are happy with it and then we start a round of testing / tire kicking. As discussed an rc merge is ok as the deadline has just snuck by and this is very much building on work in progress and will make the process of testing it much easier

[colemanw]

Ok merging it.
There's a few loose ends I'd like to tie up, such as supporting multiple levels of join. Will try to put together a folloup PR soon.