Make `create_client()` return a singleton client

[1st1]

We've discussed this multiple times, time to do it.

create_client() and create_async_client() should create a client once upon the first call (one sync singleton and one async singleton respectively) and follow on calls should just return that first thing.

• Add detached=False argument to force creating a new client when flipped to True
• Obviously singletons should be partitioned by connection arguments
• Use weakref.WeakValueDictionaty[ConnectionSpec, Client]

[1st1]

@elvis and @msullivan will discuss this and update this issue