Allow DIDComm protocols to use DIDs to identify agents sending/receiving messages

[Moopli]

For one-to-one agent communication, protocol code shouldn't need to include code for retrieving destination parameters when sending messages, and shouldn't need to decipher the identity of a sender when receiving messages. Instead, the outbound dispatcher or helper functions should retrieve destination parameters given a DID, and inbound transport should provide the sender's DID, so protocols can operate solely using DIDs if they don't need more customized functionality.

[Moopli]

We could also return the connection ID when we unpack - if we have a way to look up the connection record using the key from the other agent (who sent the message to you).

[Moopli]

Rewrote the issue, since it was describing one potential solution rather than the problem to be solved.

[fqutishat]

@troyronda @rolsonquadras @Moopli
#998 need this functionality