Facilitating Submission of Referenced Claims with Integrity Support using multipart/form-data

[piraces]

This PR aims for the proposal in #75.

As stated in this issue, in order to address the challenge of managing large claims, the proposal of enabling multipart/form-data for the submission of the Authorization Response in the direct_post Response mode is added.

By adding this Content-Type, we can efficiently submit and reference files from Verifiable Credentials.

In the changes an example of POST request with this Content-Type is included to let the users know how they may perform this request.

I also "tried" to add/reference the different scenarios in #75 which are for example the use of hashlinks or SRI, which (as can be read in the references of the issue) is addressed in multiple discussions.

Note: first contributor to a spec, any kind of feedback is greatly appreciated. Probably I'm missing something or doing something wrong but I'm committed with this.

[Sakurann]

thank you for doing the PR to illustrate your proposal. we have not discussed the issue/proposal in the WG, yet - putting do-not-merge label. (I have a feeling this example could have fit into the issue description..)

[jogu]

I think we need to discuss this further on the issue; I've added a comment there. I propose we close this PR in a week; we can always reopen it once a conclusion is reached on the issue.

[jogu]

Closing as per previous comment; let's continue discussion on the issue and potentially reopen once we reach a conclusion.