IdentityHub should be able to use multiple VP issuers

[thomasrutger]

Feature Request

When Requesting a VP using the PresentationApi of the IdentityHub, the iss field of the presentation is always set to what is given by the edc.ih.iam.id configuration property. In the case of having more than 1 participant that can issue VP's. It should use the did of the participant in question instead.

Which Areas Would Be Affected?

Verifiable Presentation Generation

Why Is the Feature Desired?

When using more than 1 participant, the issuer should be set to the proper participant did.

Solution Proposal

Change the interface PresentationGenerator<T>.generatePresentation to include the proper did (or `ParticipantContext(Id) so the did can be resolved). Alternatively, if there is a way to deduce this from the public or private key id, the interface does not need alteration.