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: https://github.com/openid/SIOPv2/issues/22.

Links to any relevant issues

• 72

• https://github.com/openid/SIOPv2/issues/22

How the change has been tested

Added unit test..

Definition of Done checklist

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• [x] I have performed a self-review of my own code
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• [ ] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works
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