Update auth-helper libs and improve lib usability

[DaughterOfMars]

Currently, the auth-helper crate does not expose enough of the underlying jsonwebtoken lib (it should just re-export, as it is meant to be used in place of jsonwebtoken), and some of the functionality is too restrictive. The ClaimsBuilder pattern is entirely unnecessary, and it is more useful to simply have full access to the Claims fields. Similarly, the JsonWebToken should expose the inner string.

Additionally, I have added a sealed trait (BuildValidation) to allow builder-lite functionality for the Validation struct, and accept that struct rather than a restrictive set of claims in the JsonWebToken::validate method.

jwt.validate(
    Validation::default()
        .with_issuer("issuer")
        .with_audience("audience")
        .with_subject("subject")
        .validate_nbf(true),
    b"secret"
)

Furthermore, I have removed the possibility for a panic when creating Claims due to SystemTime::duration_since, as the appropriate way to handle that is simply to default the duration (not that it should be possible for this to fail).

Lastly, prior to the changes there was no way to avoid validating the Subject field. However, this should be optional (as with the other fields), particularly because stateless interactions do not allow servers to maintain this information or validate it.