fix: Fix remaining REST transport issues

[vam-google]

This change depends on https://github.com/googleapis/python-api-core/pull/428 and needs api-core-2.9.0 to be released

This includes:

1. Use message-based transcoding logic instead of dictionary-based. This fixes multiple bugs caused by dict-json-object representation mismatches fro protobuf objects. This also removes a lot of json->dict->object->proto-plus object conversion overhead, which was slow and very error-prone. Now all transcodding and rest-transport-specific logic is executed in terms of native protobuf (_pb) stubs objects. No dicts, no proto-plus, no unnecessary conversions.
2. Fix for non-proto-plus objects in rest.py transport and in tests. This essential for APIs which have IAM methods defined on their surface.
3. Fix support for Any field in tests
4. Fix creation of mock_values (do mock_value1 instead of mock_value_1, since mock_value_1 causes issues for FieldMask mocks

[vam-google]

The showcase tests need to wait till api-core-2.9.0 gets released to pypi