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_core/src/persist/user/traits.rs

@@ -458,6 +458,7 @@ pub trait SocialUser: Send + Sync {
     async fn cancel_friend(&self, username: String) -> Result<bool, polariton_server::operations::SimpleOpError>;
     async fn remove_friend(&self, username: String) -> Result<bool, polariton_server::operations::SimpleOpError>;
     async fn list_friends(&self) -> Result<Vec<FriendData>, polariton_server::operations::SimpleOpError>;
+    async fn list_social_info(&self, public_ids: &[String]) -> Result<Vec<SocialInfo>, polariton_server::operations::SimpleOpError>;
     async fn has_unclaimed_match_rewards(&self) -> Result<bool, polariton_server::operations::SimpleOpError>;
     async fn get_unclaimed_match_rewards(&self) -> Result<MatchRewards, polariton_server::operations::SimpleOpError>;
     async fn claim_match_rewards(&self) -> Result<bool, polariton_server::operations::SimpleOpError>;
@@ -498,6 +499,13 @@ pub struct FriendData {
     pub avatar_id: u32,
 }
 
+#[derive(Clone)]
+pub struct SocialInfo {
+    pub public_id: String,
+    pub display_name: String,
+    pub avatar_id: Option<i32>,
+}
+
 pub enum FriendInviteStatus {
     InviteSent,
     InvitePending,