Delegation / permission per users

[Nuhvi]

Unlike #10 where you grant permissions to clients/apps, it might be necessary to grant permissions to users who aren't themselves on the same homeserver as you. For example, allow someone to write on a specific drive (as inbox) or a group chat, or just a delegated Keypair.

To achieve that, homeservers need to be able to authn user that isn't already registered on the homeserver, and keep track of the permissions they are granted.

This permissions/delegations could be colocated in the repos/drives themselves (to be easily migrated to other homeservers) or they could be kept in a higher level (a Merkle Treap for the whole account).