Add "labelField" metadata to entities

[colemanw]

Overview

This PR adds some metadata to the schema for which field will be shown when displaying an entity (e.g. when viewing a Contact, show the display_name).

Before

How to show an entity was determined by either hard-coding or guesswork (e.g. Search Kit attempts to guess by looking for a field named display_name, title, label or name).

After

How to show an entity can be known by looking at metadata

Comments

This PR only adds the metadata. A followup PR will make use of it once this is merged.

[civibot]

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[seamuslee001]

This makes sense to me MOP

[demeritcowboy]

Misc thought/wishlist: Depending on where it's going to be used I'm not sure "subject" is the best label for something like a Case or Activity. You often wouldn't be able to tell multiple records apart. A results page like search kit is probably ok since you could add more columns, but in a dropdown they'd all look the same. A compound label, like the Case entityRef displays, is better in a dropdown.

[colemanw]

@demeritcowboy the current use is Search Kit where columns are all shown separately, but I agree it's not enough metadata for a dropdown select widget for some entities.
It's a start though.