improvement: send 200 response for turn server info

We didn't implement it, but this will stop clients from retrying the
endpoint every minute

src/client_server/voip.rs

-use crate::{ConduitResult, Error};
-use ruma::api::client::{error::ErrorKind, r0::message::send_message_event};
+use crate::ConduitResult;
+use ruma::api::client::r0::voip::get_turn_server_info;
+use std::time::Duration;
 
 #[cfg(feature = "conduit_bin")]
 use rocket::get;
 
 #[cfg_attr(feature = "conduit_bin", get("/_matrix/client/r0/voip/turnServer"))]
-pub async fn turn_server_route() -> ConduitResult<send_message_event::Response> {
-    Err(Error::BadRequest(
-        ErrorKind::NotFound,
-        "There is no turn server yet.",
-    ))
+pub async fn turn_server_route() -> ConduitResult<get_turn_server_info::Response> {
+    Ok(get_turn_server_info::Response {
+        username: "".to_owned(),
+        password: "".to_owned(),
+        uris: Vec::new(),
+        ttl: Duration::from_secs(60 * 60 * 24),
+    }
+    .into())
 }