python(refactor): port grpc transport modules into sift_client

[wei-qlu]

What changed

sift_client was depending on sift_py internals for its gRPC transport layer (e.g., grpc_transport.py was importing SiftChannelConfig and use_sift_async_channel directly from sift_py/grpc. Since sift_py is being deprecated, this dependency should be removed and sift_client should be self-contained.

The gRPC transport modules were ported from sift_py/grpc to sift_client/_internal/grpc/, keeping the same structure. grpc_transport.py was then updated to import from the new internal location instead of sift_py (keeping sift_py untouched).

Verification

Ran transport_test.py manually against the new internal modules and both tests passed. Manually tested the connection through a local script using the library, verifying the SiftClient connection.

[wei-qlu]

Working on fixing the ruff check errors