feat(gateway): add cache event handler stubs

gateway/src/cache.rs

@@ -0,0 +1,73 @@
+use twilight_gateway::Event;
+
+pub async fn handle_event<'a>(
+    event: Event,
+    rconn: redis::Client
+) -> anyhow::Result<()> {
+    match event {
+        // todo: SaveDMChannelStub (?)
+        // todo: save user profiles for some reason (from message create, etc)
+        // todo(dotnet): remove relying on cache.OwnUserId
+
+        Event::GuildCreate(guild) => {
+            // save guild itself
+            // save all roles in guild
+            // save guild-role map
+            // save all channels in guild
+            // save all threads in guild (as channels lol)
+            // save guild-channel map
+            // save self guild member
+            
+            // c# code also saves users in guildCreate.Members, but I'm pretty sure that doesn't have anything now because intents
+        }
+        Event::GuildUpdate(guild) => {
+            // save guild itself
+        }
+        Event::GuildDelete(guild) => {
+            // delete guild
+            // this probably should also delete all channels/roles/etc of the guild
+        }
+        Event::MemberUpdate(member) => {
+            // save self guild member
+        }
+        Event::ChannelCreate(channel) => {
+            // save channel
+            // update guild-channel map
+        }
+        Event::ChannelUpdate(channel) => {
+            // save channel
+        }
+        Event::ChannelDelete(channel) => {
+            // delete channel
+            // update guild-channel map
+        }
+        Event::RoleCreate(role) => {
+            // save role
+            // update guild-role map
+        }
+        Event::RoleUpdate(role) => {
+            // save role
+        }
+        Event::RoleDelete(role) => {
+            // delete role
+            // update guild-role map
+        }
+        Event::ThreadCreate(thread) => {
+            // save channel
+            // update guild-channel map
+        }
+        Event::ThreadUpdate(thread) => {
+            // save thread
+        }
+        Event::ThreadDelete(thread) => {
+            // delete channel
+            // update guild-channel map
+        }
+        Event::ThreadListSync(tls) => {
+            // save channels
+        }
+        _ => {}
+    }
+
+    Ok(())
+}