Abstract register form logic into plugins

Problem/Motivation

The form logic for registering users (including anonymous) is not elegant, and is difficult to extend to other entity types, as in #1653458: Register an entity different than a user.

Proposed resolution

Create a new plugin type for registration forms, and move some of the current code in RegisterForm to one or more plugins handling different types of user registrations. The new plugin type should have "entity_types" metadata allowing the form to specify what entity types it applies to.

Remaining tasks

Develop an MR with this approach. It needs to be fully BC for the existing install base, which should not be a problem.

User interface changes

This should not be visible to an end user who is registering. It simply modernizes the approach so that registering other entity types besides users is easier.

API changes

A new plugin type will be created.

Data model changes

None.