Add dynamic client management

[brockallen]

Following up from the DCM feature.

https://datatracker.ietf.org/doc/html/rfc7592

[brockallen]

Or we can consider dynamic scope/resource registration (instead of management). Then we can consider management of all of those afterwards (e.g. for a 7.1 or some later version).

So first thing is to decide which.

[brockallen]

Also, might be interesting to have a look at the work Damian did here just to get some ideas or see if anything might be useful.

[brockallen]

We have decided to push this to 7.1.