Add async client

[andystaples]

Adds an async version of TaskHubGrpcClient (AsyncTaskHubGrpcClient) that offers async-aware client methods using grpc.aio's async gRPC calls. Refactors code shared by both clients to /internal/client_helpers.py.
Compare to earlier open-source contrib #65 / dapr#17
Resolves #64

[Copilot]

Pull request overview

Adds an asyncio-friendly gRPC client to the Durable Task Python SDK (AsyncTaskHubGrpcClient) built on grpc.aio, and refactors shared request/interceptor/channel logic into internal helpers to keep sync and async clients aligned.

Changes:

• Introduces AsyncTaskHubGrpcClient with async equivalents of orchestration/entity operations.
• Adds async gRPC channel creation (get_async_grpc_channel) and async metadata interceptor support.
• Adds async E2E coverage and updates test dependencies/configuration to run async pytest tests.

Reviewed changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
durabletask/client.py	Adds AsyncTaskHubGrpcClient and refactors shared logic usage for sync client.
durabletask/internal/client_helpers.py	New shared request/interceptor helper functions used by both clients.
durabletask/internal/shared.py	Adds get_async_grpc_channel and async interceptor typing.
durabletask/internal/grpc_interceptor.py	Adds DefaultAsyncClientInterceptorImpl and shared metadata application helper.
tests/durabletask/test_orchestration_async_e2e.py	New async E2E tests validating async client behavior with a sidecar.
tests/durabletask/test_client.py	Adds unit tests for async channel creation and async client construction.
requirements.txt	Adds pytest-asyncio for async testing.
pyproject.toml	Configures pytest-asyncio asyncio_mode = "auto".
CHANGELOG.md	Documents the new async client and related helpers.