impl MSC2965: self-advertise as OIDC authentication provider

MSC2965 proposes to let the homeserver advertise its current OIDC authentication
issuer. These changes let conduwuit advertise itself as the issuer when
[global.auth.enable_oidc_login] is set. It also advertises its account management
endpoint if [global.auth.enable_oidc_account_management] is set.

None of these endpoints are implemented. This commit only implements the bare
advertisement, as requested by the MSC.

src/api/router.rs

@@ -115,6 +115,9 @@ pub fn build(router: Router<State>, server: &Server) -> Router<State> {
 		.ruma_route(&client::get_protocols_route)
 		.route("/_matrix/client/unstable/thirdparty/protocols",
 			get(client::get_protocols_route_unstable))
+		// MSC2965 is still not stabilized. See https://github.com/sandhose/matrix-spec-proposals/blob/msc/sandhose/oidc-discovery/proposals/2965-auth-metadata.md#unstable-prefix
+		.route("/_matrix/client/unstable/org.matrix.msc2965/auth_metadata",
+			get(client::get_auth_metadata))
 		.ruma_route(&client::send_message_event_route)
 		.ruma_route(&client::send_state_event_for_key_route)
 		.ruma_route(&client::get_state_events_route)