feat: Handler to check azp claim

[gkats]

The JWT verification function accepts a handler that can be used to validate the 'azp' claim of the token.

The WithHeaderAuthorization middleware accepts either a handler or a list of authorized parties that must contain the 'azp' claim.

You can use the option in two ways.

func checkAZP(azp string) bool {
  // do whatever you want with the azp claim
}

WithHeaderAuthorization(AuthorizedParty(checkAZP))
// OR
// This will check that the 'azp' claim is one of the provided values
WithHeaderAuthorization(AuthorizedPartyFunc("clerk.com", "clerk.dev"))

[yourtallness]

As discussed in Slack, this personally confuses me.

Maybe a naming that conveys what it does (check containment in list) instead of a name that coincides with the param below.

[gkats]

@yourtallness I made a couple of changes in a23a391

• Renamed the verification parameter from AuthorizedPartyFunc to AuthorizedPartyHandler
• Renamed the option helper in the middleware from AuthorizedPartyFunc to AuthorizedPartyMatches

Please let me know what you think, thanks!

[yourtallness]

This is an improvement.

The ideal UX would probably automatically call the default matcher if no handler is provided, but I understand this may be impractical.

So, approving AFAIAC.