fix: fix serde for "id_token vp_token"

[nanderstabel]

Description of change

ResponseType::IdTokenVpToken incorrectly serialized to id_token_vp_token instead of to id_token vp_token

Links to any relevant issues

n/a

How the change has been tested

Added one simple unittest called: test_response_type_serde

Definition of Done checklist

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• [x] I have followed the contribution guidelines for this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes