Use JWT credentials for Domain Linkage

[PhilippGackstatter]

Description of change

Updates the domain linkage types to use JWT-based credentials.

Noteworthy changes:

• test_multiple_subjects was removed as it is not possible to create a JWT with multiple subjects since the "sub" claim of a JWT is of type string rather than array.
• Replace Issuer type with CoreDID on the issuer, since the spec prescribes:

  • credentialSubject.id MUST be present, and MUST be a DID.

• Validate that origin is a domain.

Links to any relevant issues

part of #1103

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

Domain Linkage tests were updated.

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

[abdulmth]

Looks good, minor comments, thanks for working on it.