[moetemp]

This PR vendors the Stream service WIT and generates its Python bindings with nexgen, into temporalio/streams/system/. It also adds the writer client in temporalio/streams/writer_system/, written by hand.

• The WIT is the file of feat(brook): add the Stream service messages (as experimental) temporalio/api#895 at 6bdefacc, except that the copy keeps the TypeScript and .NET poll as a TODO comment, as nexgen's sample does, until nexgen supports them. nexgen reads the Stream service protos from the Core submodule, and generates the client target and the Workflow target. The Stream resource has append, read, close and describe, and a read item is the StreamItem union of StreamItemData and StreamItemSuperseded.
• The payload type is a type parameter of the generated models and resources, so the bindings need no definition class. A generic call takes it as a subscript, as in create_stream[Turn](...), get_stream[Turn](...) and read_records[Turn](...).
• A create takes the fields of its StreamRef (id, owner and backend) and an id conflict policy, and the factory get_stream takes the same fields. A call that returns the Stream returns a ResourceCall, so async with closes it. Stream.append(*records) returns the last offset through the new last_offset() support helper, and client calls take rpc_metadata and rpc_timeout. Its request id comes from the new request_id() source in temporalio.nexus.system._support, which is workflow.uuid4() in a Workflow, so a replay and a task retry send the same id, and a random UUID elsewhere. In Workflow code, [Brook S7] Added Workflow Streams, which write through Local Activities. #377 sends an id that's the same in each run of the chain instead, and the source returns an empty id there.
• The workflow_writer() and client_writer() sources fill the writer of an append and a close from where the call runs. The Workflow sources raise until Workflow code can write in [Brook S7] Added Workflow Streams, which write through Local Activities. #377. client_writer raises, since a generated call doesn't say which client makes it.
• The bindings have no error classes. A failed call raises what the Nexus client raises, and [Brook S3] Added Stream refs, Records and errors in temporalio.streams. #373 turns it into a Stream error.
• The writer has no WIT. writer_system/ keeps the shape of nexgen's client output by hand, with the generated StreamRef, Producer, StreamWriter and epoch models and the coresdk.streams messages of [Brook B1] Added the streams bridge feature that links the temporalio-streams crate. #370. Like the generated calls, it maps no error. So a later switch to generated code changes no caller.
• read-records has @nexus.poll. iter_read yields pages, and iter_read_items yields items with the request to resume from. After a Superseded item, that request starts at the item's own offset, since the first Record of its batch has that offset. A read ends when the Stream is closed and the read gave its last item.
• poe gen-protos runs the generator, and check-protos installs the pinned nexgen, so drift fails CI.
• The Workflow Service bindings move to nexgen 0.2.11, the change of fix: send unknown System Nexus operations to the generic interceptor hook temporalio/sdk-python#2001. Its dispatch passes a System Nexus call that it didn't generate to start_system_nexus_operation on the Workflow outbound interceptor. [Brook S7] Added Workflow Streams, which write through Local Activities. #377 serves the Stream calls of Workflow code through that hook.
• Fork-only pin. nexgen is built from moetemp/nexgen at a3842089, which combines feat: make a resource generic in the type parameters it uses temporalio/nexgen#358, [Brook G3] Added activity.Info.workflow_task for Local Activities. #369, [Brook S2] Generated the Stream service bindings with nexgen, and added the writer client. #372, [Brook S3] Added Stream refs, Records and errors in temporalio.streams. #373, [Jetty] Merged Brook #365 and made native the default Stream backend. #366, [Brook G2] Converted transfer types nested in JSON values. #368, [Brook S1] Added the Redis Store's config proto and its generated code. #371 and [Brook] Added StreamClient. #345 with the stream sample sync of chore(samples): sync the stream sample with the latest temporalio/api#895 temporalio/nexgen#374. Upstream installs a nexgen release by --version once they merge.

Why?

The Streams API calls the Stream service through generated bindings, as the SDK calls the Workflow Service System Nexus operations. They're generated, so they get their own PR, before the hand-written Stream code. The writer client is here too, since it has the same shape.

Part of the Streams review series.

Checklist

1. How was this tested:
   • A regeneration with scripts/gen_stream_service.py and the pinned nexgen leaves no diff. poe lint, poe gen-docs and the binding tests pass.

[moetemp]

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