Present Proof V2.0 message structures

[gmulhearn-anonyome]

Related, almost identical to: #990

NOTE: I've gone with v2.0 (as opposed to v2.1 & v2.2), this is primarily because it seems to be what others (aca-py & AFJ) are targeting and expect. similarly, V2.0 is what AIP2.0 lists as it's requirement: https://github.com/hyperledger/aries-rfcs/blob/main/concepts/0302-aries-interop-profile/README.md#base-requirements

The changes are virtually identical to #990 , other changes include:

• moved AttachmentFormatSpecifier to a common place

[codecov-commenter]

Codecov Report

Merging #1017 (92e8ed9) into main (787b51c) will decrease coverage by 0.22%.
The diff coverage is 22.18%.

@@            Coverage Diff             @@
##             main    #1017      +/-   ##
==========================================
- Coverage   36.52%   36.31%   -0.22%     
==========================================
  Files         378      386       +8     
  Lines       21912    22040     +128     
  Branches     4035     4062      +27     
==========================================
  Hits         8003     8003              
- Misses      11751    11877     +126     
- Partials     2158     2160       +2     

Flag	Coverage Δ
unittests-aries-vcx	36.31% <22.18%> (-0.22%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...ation/verifier/states/presentation_request_sent.rs	53.84% <100.00%> (ø)
aries_vcx/tests/test_credential_retrieval.rs	88.67% <100.00%> (ø)
aries_vcx/tests/utils/scenarios/data.rs	96.49% <ø> (ø)
...lds/protocols/cred_issuance/v2/issue_credential.rs	0.00% <ø> (ø)
...s/src/msg_fields/protocols/cred_issuance/v2/mod.rs	0.00% <ø> (ø)
...lds/protocols/cred_issuance/v2/offer_credential.rs	0.00% <ø> (ø)
...s/protocols/cred_issuance/v2/propose_credential.rs	0.00% <ø> (ø)
...s/protocols/cred_issuance/v2/request_credential.rs	0.00% <ø> (ø)
...c/msg_fields/protocols/present_proof/v1/present.rs	25.00% <ø> (ø)
...c/msg_fields/protocols/present_proof/v1/propose.rs	16.00% <ø> (ø)
... and 36 more

[bobozaur]

LGTM. Feel free to ignore the comment posted if it's not important for you.