Support addition of exclusion URLs to the JWT auth

[ignatij]

Checklist

• I agree to the terms within the Auth0 Code of Conduct.

Describe the problem you'd like to have solved

We have a server that's accessed from the public network but also provides an API for a machine-to-machine communication. For this API there's no need for the JWT auth process, so ideally we would like to exclude these URLs from the authorization.

Describe the ideal solution

Another middleware that will be provide an easy and intuitive approach for excluding some URLs of the JWT auth process. For example something similar to the current error handler but maybe adapted a bit:

	exlcusionUrlHandler := func(w http.ResponseWriter, r *http.Request) bool {
		if isExclusionUrl(r.URL.String()) {
                      return true
                 }
                 return false
	}
     jwtmiddleware.New(
		jwtmiddleware.WithExclusionUrl(exlcusionUrlHandler),
	)

Alternatives and current workarounds

Currently we use the following approach to achieve this (not sure if it's the best way though):

        jwtExclusions := []string{...}
	jwt := func(next http.Handler) http.Handler {
		fn := func(w http.ResponseWriter, r *http.Request) {
			if isJwtExclusion(r.URL.String(), jwtExclusions) {
				next.ServeHTTP(w, r)
				return
			}
			middleware.CheckJWT(next).ServeHTTP(w, r)
		}
		return http.HandlerFunc(fn)
	}

Additional context

More than happy to provide a PR for this feature if you agree with the provided use case.