Add conformance tests for Ed25519 signatures

[henrylyons]

What is the areas you would like to add the new feature to?

Go-COSE Library

Is your feature request related to a problem?

The project README states that this module supports Ed25519 signatures - however, there are no conformance tests for this. Please add conformance tests for these types of these signatures so that they can use to verify the correctness of the Ed25519 implementation in this module, and modules written in other languages as well (e.g., Rust).

What solution do you propose?

Add positive and negative conformance tests for Ed25519 signatures, covering signature creation and verification functionality.

What alternatives have you considered?

I can create separate Ed25519 conformance tests for my COSE implementation, but I prefer to have a common set of tests that can be used to test COSE implementations in different languages.

Any additional context?

No response

[OR13]

https://datatracker.ietf.org/doc/html/rfc8152#section-8.2

Not sure if there are test vectors that we can import, but we should start by finding and linking them.

[OR13]

Blocked pending: gluecose/test-vectors#15

[OR13]

Unblocked I think, see: gluecose/test-vectors#15 (comment)