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/operations/friend_remove.rs

@@ -0,0 +1,34 @@
+use polariton_server::operations::{SimpleOpError, SimpleOperation, SimpleOpImpl};
+use polariton::operation::{ParameterTable, Typed};
+
+const CODE: u8 = 3;
+
+const USERNAME_PARAM_KEY: u8 = 1; // str; in & out
+
+pub(super) struct FriendRequestRemover {
+    social: std::sync::Arc<crate::SocialMesh>,
+}
+
+#[async_trait::async_trait]
+impl SimpleOperation<crate::data::custom::CustomType> for FriendRequestRemover {
+    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()?;
+            user_info.remove_friend(username.string.clone()).await?;
+            self.social.send_event_to(&username.string, crate::events::friend_removed::FriendRemoved {
+                friend_public_id: user_info.public_id().to_owned(),
+                friend_display_name: user_info.display_name().to_owned(),
+            }).await;
+        }
+        Ok(params)
+    }
+}
+
+pub(super) fn friend_remove_provider(init_ctx: &crate::InitConfig) -> SimpleOpImpl<crate::data::custom::CustomType, crate::UserTy, FriendRequestRemover> {
+    SimpleOpImpl::new(FriendRequestRemover {
+        social: init_ctx.social.clone(),
+    })
+}