feat: add support for using multiple DID methods

[nanderstabel]

Description of change

This change adds the functionality of using multiple different DID methods through one single Subject.

Links to any relevant issues

impierce/identity-wallet#94

How the change has been tested

Added the new MultiDidMethodSubject struct to tests/siopv2/implicit.rs and added #[rstest::rstest] to test_implicit_flow. This way test_implicit_flow is tested twice, once for did:key and once for did: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

[daniel-mader]

Adding support for a new feature while having a negative total amount of lines changed ... I like that! 😉