Platoons social functionality (#91)

### Description

This implements and completes #87 (to the best of my knowledge)

### 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/91
Co-authored-by: NG (Graham) <ngniusness@gmail.com>
Co-committed-by: NG (Graham) <ngniusness@gmail.com>

rc_social_room/src/operations/user_can_be_platooned.rs

@@ -0,0 +1,52 @@
+use polariton_server::operations::{SimpleOpError, SimpleOperation, SimpleOpImpl};
+use polariton::operation::{ParameterTable, Typed};
+
+#[allow(dead_code)]
+#[repr(u8)]
+enum ResponseCode {
+    Ok = 0,
+    InCustomGame = 1,
+    OutstandingCustomInvite = 2,
+    Error = 3,
+    PlayerDoesNotExist = 4,
+}
+
+const CODE: u8 = 59;
+
+const USERNAME_PARAM_KEY: u8 = 65; // str; in
+const RESPONSE_CODE_PARAM_KEY: u8 = 66; // bool; out
+
+pub(super) struct UserCanBeInvitedToPlatoonGetter {
+    social: std::sync::Arc<crate::SocialMesh>,
+}
+
+#[async_trait::async_trait]
+impl SimpleOperation<crate::data::custom::CustomType> for UserCanBeInvitedToPlatoonGetter {
+    type User = crate::UserTy;
+    const CODE: u8 = CODE;
+
+    async fn handle(&self, mut params: ParameterTable<crate::data::custom::CustomType>, user: &Self::User) -> Result<ParameterTable<crate::data::custom::CustomType>, SimpleOpError> {
+        if let Some(Typed::Str(username)) = params.remove(&USERNAME_PARAM_KEY) {
+            let _user_info = user.user()?; // just to validate request is authenticated
+            let mut set = std::collections::HashSet::with_capacity(1);
+            set.insert(username.string.clone());
+            self.social.filter_online_only(&mut set).await;
+            if set.is_empty() {
+                params.insert(RESPONSE_CODE_PARAM_KEY, Typed::Int(ResponseCode::Error as i32));
+                return Ok(params);
+            }
+            /*if self.social.platoon_of_user(&username.string).await.is_some() {
+                params.insert(RESPONSE_CODE_PARAM_KEY, Typed::Int(ResponseCode::OutstandingCustomInvite as i32));
+                return Ok(params);
+            }*/
+            params.insert(RESPONSE_CODE_PARAM_KEY, Typed::Int(ResponseCode::Ok as i32));
+        }
+        Ok(params)
+    }
+}
+
+pub(super) fn can_invite_to_platoon_provider(init_ctx: &crate::InitConfig) -> SimpleOpImpl<crate::data::custom::CustomType, crate::UserTy, UserCanBeInvitedToPlatoonGetter> {
+    SimpleOpImpl::new(UserCanBeInvitedToPlatoonGetter {
+        social: init_ctx.social.clone(),
+    })
+}