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/client/oidc/mod.rs

@@ -0,0 +1,3 @@
+mod discovery;
+
+pub(crate) use self::discovery::get_auth_metadata;