Update method spec and JWK method type

[PhilippGackstatter]

Description of change

Update the method spec and JWK method type.

• Method spec previously incorrectly allowed for a network name to be of length 0. If present, its length must be > 0. This is already correctly checked in code. The code also allows for the presence of digits in the network name and the spec was updated to match that, too.
• Use JsonWebKey as the method type according to Verification Method type used with VC-JWT w3c/vc-jose-cose#83, as it is the best recommendation we have for now.

Links to any relevant issues

n/a

Type of change

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• 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

None.

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