feat: introduce utoipa feature, document TxCodeConstraints schema

[daniel-mader]

Description of change

• Adds a new feature utoipa (off by default).
• Add ToSchema annotations on structs that are exported by callers (such as ssi-agent).

Links to any relevant issues

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How the change has been tested

Describe the tests that you ran to verify your changes.
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Definition of Done checklist

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• 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

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
oid4vci/src/credential_offer.rs	100.00% <100.00%> (ø)

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