Add initial support for verifiable presentations

[nanderstabel]

Description

Add initial support for verifiable presentations.

This Issue involves adding support for the vp_token response_type and presentation_definition parameter in the SIOP library, as described in the OpenID for Verifiable Presentations (OpenID4VP) specification. When the vp_token response type is set, the Provider should generate a presentation of verifiable credentials based on the presentation_definition parameter in the request. The Relying Party should then be able to verify the presentation and authenticate the user based on the claims presented.

This implementation should initially be following the IOTA DID method.

Motivation

Apart from the SIOPv2 specification, this library should also be able to support the OpenID for Verifiable Presentations specification. The initial implementation as described above will also allow for testing this library against external Client libraries.

Resources

OpenID for Verifiable Presentations (OpenID4VP) specification
#4

To-do List

• Add a PresentationDefinition struct that can be deserialized from a SiopRequest
• Add a VerifiablePresentation struct that can be build from a PresentationDefinition
• Provide a presentation_submission parameter alongside the new vp_token in the SiopResponse
• Verifiable Presentations functionality should be behind a openid4vp feature flag