radicle: Implement remote_refs_at

Implement this function on `RemoteRepository`, and move `repositories`
to the trait level.

This function will be used to quickly get the `RefsAt` of all repository
remotes.

radicle-httpd/src/api/v1/stats.rs

@@ -4,6 +4,8 @@ use axum::routing::get;
 use axum::{Json, Router};
 use serde_json::json;
 
+use radicle::storage::ReadStorage;
+
 use crate::api::error::Error;
 use crate::api::Context;