Change verifiable_credential to type Vec<CRED> in Presentation

[nanderstabel]

Description of change

This change ensures that verifiable_credential in Presentation will serialize into a [T] instead of a T, which was previously the case when verifiable_credential would only hold one single credential.

This previous serialization is in line with how credentialSubject can be serialized: https://www.w3.org/TR/vc-data-model/#credential-subject, however this is not explicitly stated for verifiableCredential: https://www.w3.org/TR/vc-data-model/#presentations-0.
The following examples that are used in the specification seem to imply that 'single credentials' should be serialized as [<credential>]:

• https://www.w3.org/TR/vc-data-model/#example-a-simple-example-of-a-verifiable-presentation
• https://www.w3.org/TR/vc-data-model/#example-usage-of-the-termsofuse-property-by-a-holder
• https://www.w3.org/TR/vc-data-model/#example-a-verifiable-presentation-that-supports-cl-signatures

The W3C VC Data Model 2.0 specification fortunately is more specific in this regard: https://www.w3.org/TR/vc-data-model-2.0/#presentations-0:

verifiableCredential
The verifiableCredential property MAY be present. The value MUST be an array of one or more verifiable credentials, or of data derived from verifiable credentials in a cryptographically verifiable format.

Therefore the introduction of the change in this PR.

Important detail:
Both W3C VC Data Model versions state that: MAY be present and **If** present..., which imply that verifiableCredential is an optional property. I fail to think of any use case where one would have a presentation without any credentials included but this optional status of verifiableCredential is included in this PR as well.

Links to any relevant issues

n/a

Type of change

Add an x to the boxes that are relevant to your changes.

• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

How the change has been tested

• Added unit test: test_presentation_deserialization which tests deserializing from an example verifiable presentation (with some minor adjustments): https://www.w3.org/TR/vc-data-model/#example-a-simple-example-of-a-verifiable-presentation
• Added unit test: test_presentation_deserialization_without_credentials which tests deserializing a verifiable presentation as well, but one without any verifiableCredential.
• Added unit test: test_presentation_builder_valid_without_credentials which tests building a Presentation without any credentials set.

Change checklist

Add an x to the boxes that are relevant to your changes.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[PhilippGackstatter]

Looks good to me. Thanks again 🙏.

I'll approve this with additional comments to mean that I leave it to your judgement whether to implement them or not.

[abdulmth]

Looks good to me, thanks for putting in the effort.

[PhilippGackstatter]

The Wasm examples CI workflow ran for 2:50h but didn't finish until then so I cancelled it. Had this before at some point but a rerun did the trick back then. I haven't restarted in case you want to fix the comment, otherwise just rerun the workflow.

[nanderstabel]

@PhilippGackstatter @abdulmth thank you both for reviewing! I made the last suggested change to the unit test. Hopefully the CI workflow won't have any issues :)

[PhilippGackstatter]

@nanderstabel Our convention is to let the PR creator hit the merge button, so feel free to go ahead whenever you want. Thanks again 🚀!

[nanderstabel]

Great, thanks again!