add discord membercount service & endpoint

a background worker is added that sends an approximate member count
request every 60 seconds, caches it into memory, and serves the cache on
a public endpoint

src/discordbot/member_count/mod.rs

@@ -0,0 +1,22 @@
+use crate::discordbot::MAIN_GUILD_ID;
+use serenity::all::{Context, GuildId};
+use service::run_membercount_service;
+use tracing::info;
+
+mod service;
+
+pub use service::get_member_count;
+
+pub fn init_service(ctx: &Context, guild_id: &GuildId) {
+    let (ctx, guild_id) = (ctx.clone(), guild_id.clone());
+    info!("Initialising discord member count service...");
+
+    if guild_id != MAIN_GUILD_ID {
+        info!("Guild member count service not initialised; Bot not running on main guild.");
+        return;
+    }
+
+    tokio::spawn(async move {
+        run_membercount_service(ctx, guild_id).await;
+    });
+}