An idea for the endorsement of a badgeclass

A possible extension to allow external organizations to endorse an issuer's badgeClass by providing a badgeClass extension with a JSON Web Signatures-signed block. This is an extension to allow issuers to solicit endorsements, which wouldn't work for endorsers to add their endorsement without the issuer's permission. I'm not absolutely sure just _what_ should constitute the payload of the signature. This example suggests signers embed what they know of the badgeClass in the signature extension, copying the existing properties from above. It's not perfect. 

If signers know the URL of the badgeClass-to-be-endorsed, they could include that, but that wouldn't be enough to protect against the badgeClass being edited to become something that the signer didn't want to endorse. 

Potential downside: The only way to revoke this kind of signature without cooperation of the issuer would be to pull the public key.