Add support for relative references in core document

[wulfraem]

Description of change

Adds support for relative references in CoreDocument for OrderedSet<MethodRef> fields:

• authentication
• assertion_method
• key_agreement
• capability_delegation
• capability_invocation

Added a new enum variant MethodRef::RelativeRefer, which holds a full (including the document's id) DIDUrl. id value is inserted during deserialization andomitted during serializtion to keep DIDUrl value valid but serialize back to original value.

Links to any relevant issues

Implements #1072.

Type of change

• [ ] Bug fix (a non-breaking change which fixes an issue)
• [x] 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

New tests for (de)serialization added, existing tests still work.

Change checklist

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

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