Search kit joins

[colemanw]

Overview

This adds support for multiple and multi-layered joins in Search Kit.

Before

Ability to join once from the main entity to related entities.

After

• Ability to join two entities more than once (ex. joining from Contact to Email twice, once for home email, once for work email).
• Ability to join onto joins multiple levels down. (ex. joining from Activity to Contact to Contribution to Line Item)

Technical Details

It's cool.

[civibot]

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[eileenmcnaughton]

This code change only touches search kit - which we are still treating as beta/fast-moving & relates to already merged-to-5.33 code - so merging