[http-client-python] backcompat with legacy transport

[l0lawrence]

Storage blobs has tests that assert there is back compat with legacy transports, here

These tests fail with the new typespec generated code because the MockLegacyTransport does not have iter_bytes()

[l0lawrence]

FAILED tests\test_transports_async.py::TestStorageTransportsAsync::test_legacy_transport_old_response - AttributeError: 'AioHttpTransportResponse' object has no attribute 'iter_bytes'
FAILED tests\test_transports_async.py::TestStorageTransportsAsync::test_legacy_transport_old_response_content_validation - AttributeError: 'AioHttpTransportResponse' object has no attribute 'iter_bytes'