feat: support for vc+sd-jwt

[Zicchio]

This pull request closes #269 and #255.
Proposals and modifications are very welcomed.

The followings steps where done:

1. define a new interface for general purpose VP token (which is not intended to be a definitive and final iteration; open for review if/when we handle mdoc-cbor tokens); previous iteration "kinda" assume that the only possible VP tokens were vp-envolpes.
2. new interface is intended to be decoupled from trust-layer and direct post processing as much possible for extensibility purposes.
3. define an implementation for that interface for vc+sd-jwt tokens - In this PR only PIDs are obtained this way.
4. integrate the usage of the new interface in the response_handler.py, with some care regarding extensibility and possible modifications at the trust layers

The final iteration resulted in more modifcations that intended - special care should be taken before pulling in @peppelinux .

Note that in the process we "lost" VP-envolpe; which was expclicity against the intent of this pull request. This is because the envolpe hypotesys was spread around in multiple methods and I found it hard to salvage (I don't deny that it might be a skill issue). Hopelly, the new interface should promote a new implementation in short time.

[peppelinux]

Good achievements, anyway we have decided to continue in this PR these two core components:

• trust layer abstraction, according to: [Satosa Backend] Trust layer abstraction and extensions #267
• credential format abstraction, following the comments within this PR

[Zicchio]

dev note: I rebased the branch based on the changes of PR #270 and PR #272 and handles some conflicts (mostly in the ingration test hanges with PR #272).
no further changes are introduced