feat(did-comm) Support DIDComm Messaging attachments

[codynhat]

What issue is this PR fixing

closes #612

What is being changed

This adds support for attachments in DIDComm messages. The DIDCommMessageHandler will pass along the attachments after unpacking.

Quality

Check all that apply:

• I want these changes to be integrated
• I successfully ran yarn, yarn build, yarn test, yarn test:browser locally.
• I allow my PR to be updated by the reviewers (to speed up the review process).
• I added unit tests.
• I added integration tests.
• I did not add automated tests because _________, and I am aware that a PR without tests will likely get rejected.

[codecov]

Codecov Report

Merging #1087 (f176b4d) into next (125bf42) will decrease coverage by 0.50%.
The diff coverage is 77.30%.

Additional details and impacted files

@@            Coverage Diff             @@
##             next    #1087      +/-   ##
==========================================
- Coverage   80.25%   79.74%   -0.51%     
==========================================
  Files         118      132      +14     
  Lines        4056     4740     +684     
  Branches      875     1016     +141     
==========================================
+ Hits         3255     3780     +525     
- Misses        798      960     +162     
+ Partials        3        0       -3