transport inheritance structure
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`ExecutionsTransport` is the ABC for all transports.
- public child `ExecutionsGrpcTransport` for sync gRPC transport (defined in `grpc.py`).
- public child `ExecutionsGrpcAsyncIOTransport` for async gRPC transport (defined in `grpc_asyncio.py`).
- private child `_BaseExecutionsRestTransport` for base REST transport with inner classes `_BaseMETHOD` (defined in `rest_base.py`).
- public child `ExecutionsRestTransport` for sync REST transport with inner classes `METHOD` derived from the parent's corresponding `_BaseMETHOD` classes (defined in `rest.py`).