Support dynamic client registration

[jnschaeffer]

Is your feature request related to a problem? Please describe.
While we have a management API for clients currently, it would generally be better if we supported RFC 7591 for client registration.

Describe the solution you'd like
A standards-compliant client registration endpoint would be preferable.

Describe alternatives you've considered
We have a management API for clients today so this isn't a must, but as we build it out it may be better to use standards off the shelf.