Prepare and release ACA-Py 0.4.1

[swcurran]

Please include work up until now, plus:

• required: PR Align Forward message with RFC 0094: msg must be JSON dict, not a string #240 (please merge it)
• required: PR Adjust handling of unpadded base64 input #338
• required: PR make issue-credential proposal preview optional, aligning with Aries … #336
• optional: PR revisit send-cred-def and associated non-secret record write #333
• optional: PR accommodate referents in presentation proposal preview attribute spec… #339

As well, we need the issues raised in Aries RFCs 391 resolved and included. Needed for ACA-Py are updates to the presentation verification (checking raw vs. encoded) and possibly the encoding algorithm which is being questioned.

Please monitor if anything else is needed, but the sooner the better and NLT Wednesday with at least the three required PRs.

[sklump]

The encoding algorithm has been present from day 1:

https://github.com/hyperledger/aries-cloudagent-python/blob/401a82b10a33accd391b465f902dddc98da81803/aries_cloudagent/issuer/util.py#L9

Any change to it will make every aca-py and every indy-cat-agent credential in history ineligible for use in proofs once the raw/encoding check is in place.