Introduces a new struct type which contains the token's headers and
claims as JSON values. This is constructed by validating a token and
allows library users to deal with the deserialised values as they
please.
There are multiple points in the code where a token part needs to be
deserialised (i.e. first base64-decoded, then JSON-deserialised). This
is extracted to a helper function in this commit.
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.
This makes the library slightly more "rusty". Instead of returning a
validation result which also represents potential success, use an enum
representing the error variants and the standard library's
`Result`-type to represent success/failure.
