Add support for Protected Resource Metadata (#38)

* Implement OAuth Protected Resource Metadata struct, RFC 9728

* Fix incorrect passing Accept header for WellKnownEndpoints

The Accept header was incorrectly being set as a query string component, instead of as a request header. I've added tests that actually exercise this code to ensure that the implementation is correct.

* Remove incorrect comment — client_id is definitely required

* Fix test failures due to #37

* Fix swift 6.x failures

* Fix swift 6.x failures

* Fix swift 6.x failures