dev/core#4821 - APIv4: Allow custom field to reference value in bridge joins

[colemanw]

Overview

Fixes https://lab.civicrm.org/dev/core/-/issues/4821

Before

SQL error when trying to compare a field value in the on condition of a custom field.

After

Query works correctly.

Technical Details

When part of a bridge join there are two ways to get at the id of the joined entity. E.g. when joining Activity through ActivityContact, you could access the Activity id field either via civicrm_activity.id or via civicrm_activity_contact.activity_id. Custom fields were trying to join using the normal way of civicrm_activity.id but that fails if the custom field is getting joined prior to the activity table join. So instead of joining custom fields the normal way, they need to be joined to the bridge table.

Also includes follow-up to b367821 which adds metadata about these joins so the values are discoverable and not always assumed to match the name of the entity table (usually they do, but they don't have to).

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[seamuslee001]

Seems to work for me on the PR test site and there is a Unit test