Add a `verify_prehashed` method to `identity.rs`'s JWS verifiers

[UMR1352]

Description of change

Enable pre-hash verification for EcDSA verifier. Small refactor for JWS verifiers.

Links to any relevant issues

Fixes issue #1418

Type of change

• [ ] Bug fix (a non-breaking change which fixes an issue)
• [x] Enhancement (a non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation Fix

How the change has been tested

Ran tests

Change checklist

• [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

[UMR1352]

Unfortunately implementing pre-hash verification for for our EdDSA verifier is not as straight forward. At the moment there's only one Rust crate that allows Ed2219 pre-hash verification - i.e. ed25519-dalek but the requirements to call its validate_prehashed are way too strict.

The data to be verified must be passed to the function as a type D: Digest where Digest requires 4 more traits in order to be implemented.

[UMR1352]

After some discussion with the team we decided to ditch this feature as we want the JWS verifiers we provide with the library to only support verification of spec-compliant signatures (e.g. ES256 -> P256 + SHA256 and no other hashing algorithm). We still do support the verification of custom signatures through our JwsSigner trait.