feat: allow for DidMethod with partitioned namespace

[nanderstabel]

Description of change

This change allows for DidMethod's with namespace support. For more information see #72, but in short:
now not just did:key, did:jwk and did:iota can be parsed to a DidMethod struct, but did:iota:smr and did:iota:rms are valid DidMethod's as well.

Disclaimer:
The SIOPv2 specification has a (probably unintended) too restrictive requirement regarding displaying supported DID Methods in a Providers metadata (that doesn't allow for namespaces). There is an open issue for this which will will hopefully result in this restriction to be removed: openid/SIOPv2#22.

Links to any relevant issues

• Allow for DidMethod's with partitioned namespaces #72
• Decentralized Identifier in subject_syntax_types_supported may be too restrictive openid/SIOPv2#22

How the change has been tested

Added unit test..

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