Support indieauth-metadata endpoint #119
Description
IndieAuth now supports a bundled metadata discovery mechanism, and it would be a good idea for Authl to support that for server discovery as well.
indieauth.find_endpoints needs to check for the indieauth-metadata rel, and if it exists, consume the data from there (overriding the data discovered from the original resource).
The endpoints of interest to Authl and its primary consumers are:
authorization_endpoint(for IndieAuth's login flow)ticket_endpoint(for TicketAuth support, primarily in Publ)
Additionally, if there is now an issuer, that needs to be added to the token state and verified in IndieAuth.check_callback.