OpenPGP Support

[OR13]

@kdenhartog we talked about this ages ago, but it came up on the aries call:

https://github.com/decentralized-identity/github-did/blob/master/packages/lib/src/v2/__fixtures__/encryptedMessageFor.json

I recently added Universal Resolver and Yubikey support to:

https://github.com/transmute-industries/PROPOSAL-OpenPgpSignature2019

Since PGP / JOSE / JSON-LD are all not formally related, I wanted to kickoff the converation around OpenPGP support for DIDComm.

There are lots for legacy systems and hardware which use PGP. It would be cool to establish a pattern of use for PGP and DIDComm.