Add initial PublicKeyJwk support

[olivereanderson]

Description of change

This PR makes verification material described by publicKeyJwk representable in DID Documents. See the specification.

This PR also feature gates the diff logic.

What is not included in this PR

• The new functionality introduced here has not been tested.
• There are not enough checks ensuring JWK-based Verification methods with private key components cannot be constructed.

In order to iterate faster on the upcoming features I suggest we return to these items once we have an initial implementation of the new KMS API.

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

Runs under cargo test and builds the bindings without problems.

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