Add hook_inline_entity_form_[entity|ENTITYTYPE]_prepare_form

Re-roll #3 for 10.0.x

#8 is in
tnx

Automatically closed - issue fixed for 2 weeks with no activity.

Bulk reopen.

I still like this a lot. Revisiting this after quite some time, my gut feeling is that this must not re-purpose hook_entity_prepare_form, but invent an IEF version of it. (Anyone with a good reasoning that it is not so, please explain!)

The related issue confirms that this implementation is wrong and must not be committed.
An IEF version of that hook makes sense though.

This seems to have caused some major regressions. See 🐛 Parent form entity builders run on IEF resulting in fatal errors Active

#14:
> still like this a lot. Revisiting this after quite some time, my gut feeling is that this must not re-purpose hook_entity_prepare_form, but invent an IEF version of it.

Comment #3364996-15: Parent form entity builders run on IEF resulting in fatal errors → of that issue elaboratest that brilliantly.

Updating title accordingly.