feat(gateway): split off cache into separate package, rename package to 'gateway'

myriad_rs/build.rs

@@ -0,0 +1,6 @@
+use std::io::Result;
+
+fn main() -> Result<()> {
+    prost_build::compile_protos(&["../proto/discord_cache.proto"], &["../proto/"])?;
+    Ok(())
+}