[THardy98]

What was changed

This change introduces a minimal subset of the planned Sessions API as specified in temporalio/api#885 (commit d183f54c642fe320218bc3bf8592b2533540f050).

The intention with the Sessions API is to address worker affinity use cases.

Changes:

The current API supports:

• worker options to enable a worker to serve sessions:
  Worker(session_config=WorkerSessionConfig(...))

• worker session options include a "single session" mode that allows it to serve only a single session
  SingleSessionAdmission(stable_identity=...)
  the stable identity lets the session bind to a particular identity (i.e. in the event the worker dies and a new worker with the same stable identity is provided).

• activity session options that allow an activity to run in the same session as its workflow:
  workflow.activity_session(ActivitySessionOptions(placement=UseWorkflowSession()))
  this co-locates a workflow's workflow and activity tasks on the same worker.
  Note: the activity must run on the same task queue as the workflow (otherwise, it will be routed to its own task queue)

• Replayer allows Session workflows to replay only on session-enabled workers

Usage:

The API uses a context manager to declare session scope, example usage:

@workflow.run
async def run(self, prompt: str) -> str:
    # The scope covers activities started inside it, including plugin calls.
    with workflow.activity_session(ActivitySessionOptions(placement=UseWorkflowSession())):

        # In session: inside the scope.
        result = await workflow.execute_activity(
            run_tool, prompt, start_to_close_timeout=timedelta(minutes=5),
        )

        # Different task queue: the scope is skipped, normal routing.
        await workflow.execute_activity(
            gpu_infer, result, task_queue="gpu",
            start_to_close_timeout=timedelta(minutes=10),
        )

Note
This PR is dependent on feature branches (PRs):

• Core pinned to Add stable destinations and lifetime single-session isolation sdk-rust#1687 (6e88cb53)
• API WorkerSessionOptions + ActivitySessionOptions -- Defining Sessions api#885.

Why?

An initial Python API for the Sessions API (activity affinity and single-session workers), to serve worker affinity use cases.