WIP: Support invitation messages with public DID

[mirgee]

Add support for invitation messages in the format

{
    "@type": "did:sov:BzCbsNYhMrjHiqZDTUASHg;spec/connections/1.0/invitation",
    "@id": "12345678900987654321",
    "label": "Alice",
    "did": "did:sov:QmWbsNYhMrjHiqZDTUTEJs"
}

as per Aries connection protocol RFC.