Afform - switch entity declaration to standard metadata

[colemanw]

Overview

This builds on the work in https://lab.civicrm.org/dev/core/-/issues/4999 to switch Afform over to using entityType declarations.

Before

Afforms are oddball entities because they don't have a sql table or DAO. So they existed in APIv4 only.

After

Core EntityType declarations can now handle oddball entities & Afform is one of them!

Technical Details

The only real core change here is to allow table-less entities to exist without complaining.
For any processing that involves sql tables, we just skip over them.

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