Add API endpoint to revoke delegate key for existing target

[marcofranssen]

Revocation should can only be done if the latest signed image tags are not signed by this key.

Minimum viable:

• return a HTTP error response to inform API consumer there are images signed with this key

Improvement:

• which tags are signed with this key

[marcofranssen]

Partially implemented. No checks if this key was used for the last signature. In that case a resigning with a different key should be performed so this key can safely be removed.