[moetemp]

What was changed

This PR adds the data model of the Streams API to temporalio.common. That's Stream definitions, the StreamHandle[T] data, Records, errors and the generated wire models that they carry.

• A definition subclasses StreamDefinition[T] and has @common.defn. It gives a Stream its name and its one payload type.
• StreamHandle[T] is data only. It has the fields of the temporal.api.stream.v1.StreamHandle wire model (id, owner and backend) and no calls. S5 adds temporalio.client.Stream, which makes the calls outside Workflow code, and S7 adds temporalio.workflow.WorkflowStream, which makes them in Workflow code.
• A handle converts to the proto JSON of StreamHandle through nexgen's generated converter, also as a field of a dataclass or a pydantic model, through the nested conversion of [Brook G2] Converted transfer types nested in JSON values. #368. So a Workflow can pass a handle to an Activity, a child or the next run. The type argument, such as StreamHandle[Turn], is for type checks only.
• temporalio.common also exports the generated wire models that handles, Records, Superseded items and errors carry: Execution, ExecutionType, Producer, ProducerEpoch, StreamErrorReason, StreamProducerKind, StreamSuperseded and WorkflowTaskEpoch. So code outside the SDK can build a Workflow's handle.
• The errors are the generated classes of S2, with the public names. Each class maps a case of StreamErrorReason. They have no code, and reason is the generated StreamErrorReason IntEnum. StreamConfigError stays hand-written, since it isn't a Store reason.

It depends on #372 (the generated Stream bindings) and #368.

Why?

Every later Streams PR builds on these types, and they need no backend or server. Stream handles and big-payload handles discusses the handle's wire form, and why the handle is data with no calls.

One open question. temporalio.common gets its first __all__, which changes from temporalio.common import *. Is that OK?

Part of the Brook review series.

Checklist

1. How was this tested:
   • tests/contrib/streams/test_handles.py, and test_vectors.py, which checks the ids and withdrawal rules against the temporalio-streams crate.