Friends functionality (#90)

### Description

Implements and closes #86

### Game

Robocraft

### Please confirm

- [x] I am the legal owner or represent the owner of all work submitted
- [x] I consent to my submission being added to this FOSS project
- [ ] This PR used LLMs to generate some or all of the code changes

Reviewed-on: https://git.ngram.ca/OpenJam/rc-servers/pulls/90
Co-authored-by: NG (Graham) <ngniusness@gmail.com>
Co-committed-by: NG (Graham) <ngniusness@gmail.com>

rc_social_room/src/social_services.rs

@@ -0,0 +1,59 @@
+/// Primarily keeps track of who is online so events can be sent to them.
+pub struct SocialMesh {
+    users: tokio::sync::RwLock<std::collections::HashMap<String, UserHandle>>,
+}
+
+struct UserHandle {
+    emitter: polariton_server::events::WeakEventEmitter<crate::data::custom::CustomType>,
+    is_alive: std::sync::atomic::AtomicBool,
+}
+
+impl SocialMesh {
+    pub fn new() -> Self {
+        Self {
+            users: tokio::sync::RwLock::new(std::collections::HashMap::new()),
+        }
+    }
+
+    pub async fn send_event_to(&self, public_id: &str, event: impl polariton_server::events::IntoEvent<crate::data::custom::CustomType>) -> bool {
+        let user_lock = self.users.read().await;
+        if let Some(user_handle) = user_lock.get(public_id) {
+            let is_success = user_handle.emitter.emit(event);
+            user_handle.is_alive.swap(is_success, std::sync::atomic::Ordering::SeqCst);
+            is_success
+        } else {
+            false
+        }
+    }
+
+    pub async fn add_user(
+        &self,
+        public_id: String,
+        emitter: polariton_server::events::WeakEventEmitter<crate::data::custom::CustomType>,
+    ) {
+        let mut user_lock = self.users.write().await;
+        Self::cleanup_dead_users(&mut user_lock).await;
+        user_lock.insert(public_id, UserHandle {
+            emitter,
+            is_alive: std::sync::atomic::AtomicBool::new(true),
+        });
+    }
+
+    /// Filter out offline users
+    pub async fn filter_online_only(&self, public_ids: &mut std::collections::HashSet<String>) {
+        let mut user_lock = self.users.write().await;
+        Self::cleanup_dead_users(&mut user_lock).await;
+        public_ids.retain(|public_id| user_lock.contains_key(public_id));
+    }
+
+    async fn cleanup_dead_users(users: &mut std::collections::HashMap<String, UserHandle>) {
+        users.retain(|_public_id, handle| handle.is_alive.load(std::sync::atomic::Ordering::SeqCst));
+    }
+
+    pub async fn online_count_read(&self) -> u64 {
+        let user_lock = self.users.read().await;
+        user_lock.iter()
+            .filter(|(_, handle)| handle.is_alive.load(std::sync::atomic::Ordering::SeqCst))
+            .count() as u64
+    }
+}