feat: add support for ES256 signature algorithm

[nanderstabel]

Description of change

Although did_manager is capabable of creating and storing a ES256K key, we cannot enable it since the library we use for signing (jsonwebtoken) we use in oid4vc only supports ES256 and not ES256K.

Links to any relevant issues

• Support DIIP V2.1 specifications #177

How the change has been tested

Manually tested against impierce/ssi-agent#101

In that UniCore is a description of how UniMe can be tested against UniCore.

Definition of Done 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

[maiertech]

Fine in principle, but let me have a look again after rebasing.

[maiertech]

Fine in principle, but let me have a look again after rebasing.

[nanderstabel]

I've rebased this branch (and also fixed a bug where the preferred signing algorithm was not being respected), so it is now finally ready for the final review.