[moetemp]

What was changed

This PR adds client Stream handles (ClientStream), Client.create_stream, Client.stream and Stream reads (Subscription).

• Client.create_stream is create-or-get by Stream id. On a Stream that exists, it ignores the options and returns that Stream, and on a closed one it raises StreamClosedError. It takes an optional backend name, so it doesn't depend on the Store.
• Client.stream makes no call. A ClientStream in a query result uses the client that made the query, so a reader uses the Stream that a Workflow hands out through a stream query.
• A read yields Records, Superseded items that withdraw earlier Records, and Closed last. It long-polls and starts at an offset, at END or at the newest last Records. It matches each item on the oneof of data, superseded and close. The long poll's loop, which carries the offset across empty pages, is here.
• An in-Python fake of the crate's runtime lets the facade tests run with no Store.
• _convert.py and _writer.py are here and not in S3 and S4, since their first users are here.

Depends on S4 (#374). No changelog fragment, since S9 has the Streams entry.
Part of the Brook review series.

Why?

This is the client half of the API, which S6 runs on Redis and in memory.

Checklist

1. How was this tested:
   • poe lint, poe gen-docs and pytest tests/contrib/streams.
   • Fork CI passes, except the arm jobs, since the fork has no arm runners.