did-exchange: exchange response needs to be signed

[llorllale]

We should be signing the connection response. Specifically, the connection object needs to be replaced by connection-sig.

The reason for this is that the keys shared in the initial invitation to exchange will likely NOT be the keys present in the DID shared in the connection response:

• The keys in the initial invitation to exchange will likely be ephemeral since they are sent in plaintext (by design) and because DIDComm uses keys as identifiers. Identifiers should not be revealed to outside parties.
• The party has to prove that they own the DID that they are sharing and this is a mechanism to do so in a single message (as opposed to a later challenge-response protocol).