Discovery and errors for key rotation

[jricher]

An AS may not always allow its client instances to rotate the keys, correct? Therefore,

1. the client instances might "learn" whether it's possible in advance, during ongoing negotiation with the AS
2. the specification may define a dedicated error that will basically tell the client instances, this operation isn't allowed

Originally posted by @adeinega in https://github.com/ietf-wg-gnap/gnap-core-protocol/issues/456#issuecomment-1289402016