Add ability to update the mediator url

[goncalo-frade-iohk]

Not needed for Dec 2022 demonstration.
But this will be useful for setting/changing mediators.

This is something I've been wondering. The main issue with this is if you change mediator. Any connection previously made will be basically terminated.

Keep the status of what DIDs are connected with what mediator seems more work than expected.

But it something to take into account

[goncalo-frade-iohk]

@2byrds opened a discussion for this issue #65

[goncalo-frade-iohk]

This will enable developers to manage mediators. #71