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/mod.rs

@@ -3,6 +3,7 @@ pub(super) mod account_data;
 pub(super) mod admin;
 pub(super) mod alias;
 pub(super) mod appservice;
+pub(super) mod oidc;
 pub(super) mod backup;
 pub(super) mod capabilities;
 pub(super) mod context;
@@ -46,6 +47,7 @@ pub(super) use account_data::*;
 pub(super) use admin::*;
 pub(super) use alias::*;
 pub(super) use appservice::*;
+pub(super) use oidc::*;
 pub(super) use backup::*;
 pub(super) use capabilities::*;
 pub(super) use context::*;