A Wallet -> Issuer "credential deleted" signal

[tplooker]

Related to much of the discussion raised in PR #70, there are a few situations where it is useful for a credential issuer to know when a given credential is deleted from a wallet so that the issuer can perform additional tasks like revoking the credential.

[selfissued]

Given we added the Notification Endpoint, is there a reason for this issue not to be closed on that basis?