Support key rotation and key ordering

[woutslakhorst]

Currently the VerificationMethod SQL table has a weight property that can be used to sort keys. This will allow newer keys to get preference over older keys.

Also:
Hashicorp vault and azure keyvault support keyroration where a key with the same keyname gets a new version (preventing the use of older keys). We do not use this feature.
We generate a new uuid for the backend when creating a new key.

When adding proper key rotation we probably want to make use of this backend feature. This means that a Rotate(keyname) (keyname, version) method has to be added to the backend and a RotateKey(DID, KeyFlags) error to Crypto (creates a new VerificationMethod with bigger weight using rotate on the backend)