[moetemp]

This PR adds temporalio.client.Stream, which makes the Stream calls outside Workflow code, with Client.create_stream, Client.create_or_get_stream, Client.get_stream, Client.to_stream and Stream reads (streams.Subscription).

• A call takes the payload type as a type argument, as in client.get_stream[Turn]("chat-42"), and the id is positional. Python has no type arguments on a call, so the three calls are properties whose objects take the type as a subscript and keep it for type checkers. With no subscript, the type is Any and a read gives JSON values.
• create_stream fails with StreamAlreadyExistsError if the id exists. create_or_get_stream sends USE_EXISTING and gets it, and ignores its options there. Both fail on a closed or expired Stream, and each create sends a new request id.
• get_stream and to_stream(ref, *, payload_type=None) make no call. stream.ref is the data to pass on, and stream.proto_ref is the StreamRef wire message for a field of a proto message.
• publish starts the write at the call and returns a future. The writes of one Stream keep the order of the calls. A write error that no caller got raises at the next call of the Stream, and close closes first, then raises it.
• A close carries no value, and async with closes at the block end on any exception, as the generated resources do. A close that the Store refuses counts as not done, so the block end tries it again. A close of a Stream bound to a Workflow raises StreamNotOwnerError with no call, and so does async with at __aenter__.
• rpc_timeout is the deadline of each call on every backend. The Redis and memory backends make no RPC, so they ignore rpc_metadata.
• A read yields Records and Superseded items. It long-polls and starts at an offset, at END or at the newest last Records, 1 to 1000. It ends when the Stream is closed and the read gave its last item. A read with create_wait pauses between the reads that find no Stream.
• A client Producer has a client epoch and a key for its process. A forked child makes its own key, so two children of one parent never write as one Producer.
• A read keeps the offsets that it gave of each Producer that a later Superseded item can withdraw, as runs of consecutive offsets. A client's Records are never withdrawn, so a read keeps nothing for them.
• An in-Python fake of the crate's runtime lets the facade tests run with no Store.

Why?

This is the client half of the API, which the next PR runs on Redis and in memory.

Part of the Streams review series.

Checklist

1. How was this tested:
   • poe lint, poe gen-docs and pytest tests/contrib/streams.

[moetemp]

Closed in favor of #381, the combined branch, which has this change.