ActorMove && InstanceMove activity

[CutestNekoAqua]

ActorMove: Goal is to implement a activity that federates the transferal of a Actor like a User. The private and public key of the user shall not change to keep previous messages signed validly. API endpoint changes are to be federated, the id is to be kept the same, the name can change. If the new user is already existing (it should be able to be fetched during Move at least), like in AP, the new user needs to link back to the old user using a also known as field.

InstanceMove: Notice that a whole instance is migrating to a (new) domain. This activity can also be used to migrate from one versia software to another by only changing endpoints but keeping the domain the same. it is supposed to include a array of ActorMove activities in bulk for every existing actor on the instance.

Maybe optionally let other instances repeat a received instancemove activity to all known instances, if received for the first time, so that all instances on the network get notified about it at some point. (not sure if this is necessary)