feat: Introduce validation of JWT signatures

Introduces the internal function for validating JWT signatures. The
process is relatively straightforward:

1. Create an OpenSSL signature verifier using the public key from the
   JWK.

2. Split the JWT into the data (header + claims) and signature parts.

3. Validate the data against the signature using the verifier from (1)

OpenSSL "cleanly" returns a boolean in case of an invalid signature,
but an otherwise successful operation.

This is represented differently in the returned error variant, with an
invalid signature being represented as `InvalidSignature`, and other
errors as the `OpenSSL` error variant which wraps the underlying
OpenSSL issue.

Successful validation returns an empty `Ok` result.

0 files changed, 0 insertions, 0 deletions