ability to extract signature and signed bytes from Token/UntrustedToken

[thedavidmeister]

currently can only get header and claims but not signature

i'd like to be able to use ed25519_dalek with the signature bytes and signature directly

[slowli]

Hi, @thedavidmeister! Terribly sorry for the delayed response; somehow, didn't notice the notification.

I've sketched a solution in #9. Does it correspond to what you had in mind? Feel free to review the PR; would be much appreciated.