Use a cargo workspace for better modularity

Currently, there is only a `gateway` crate, but in the future,
likely more will be added to handle db stuff, abstractions, etc once
I actually start implementing the city portal logic itself.

services/gateway/src/schema.rs

@@ -0,0 +1,8 @@
+use crate::resolvers::QueryRoot;
+use async_graphql::{EmptyMutation, EmptySubscription, Schema};
+
+pub type AppSchema = Schema<QueryRoot, EmptyMutation, EmptySubscription>;
+
+pub fn build_schema() -> AppSchema {
+    Schema::build(QueryRoot, EmptyMutation, EmptySubscription).finish()
+}