[FIX] DIDExchange request handling should require invitation key rotation

[gmulhearn-anonyome]

Currently the DIDExchange handle_request in aries-vcx has the invitation_key parameter as optional:

invitation_key: Option<Key>,

we want to make this a required field.

there are many TODO comments about this, but essentially:

• it is used to sign the did_rotate attachment (rotating from the invitation DID to a new DID)
• it is currently optional as we weren't sure how to best handle implicit invitation responses (i.e. what key is used if the invitation was implicit?!)
• however it is now more obvious that we should use the recipient key of the service that the request was sent to (as a result of implicit invitation)
• this will need updating from Optional to required up the stack (ariesvcx->agent->aath backchannel)
• AATH backchannel internals will need to be updated such that it will be able to pass the invitation key (i.e. the recipient of the packed didcomm message) into the handler