Enhancements to Email Policy embedding

Problem/Motivation

This module can embed a form GUI for editing Mailer Policy within another form, see MailerHelperInterface::renderEntityPolicy(). It's a useful feature but the workflow has some significant limitations:

1. To edit the Policy, you have to visit a separate page.
2. The embedding doesn't work when creating a new entity - you have to save the entity, then add policy.
3. The feature cannot be used without saving the Policy entity.

Proposed resolution

We should embed the Policy entity form within the other form - maybe using module inline_entity_form. When embedded in another config entity, both entities would be saved with the same submit. In the case of a temporary policy, the code should return the Policy config rather than saving it.

Remaining tasks

User interface changes

API changes

Data model changes