Type hint for query classes

[kyungeonchoi]

It's very useful to have type hints when composing a ServiceX spec in python. Type hints are nicely shown for all fields (such as Sample, dataset) but not for query as attached.

[ponyisi]

I don't see how we can really support this with the current plugin system. If we switch to "namespace packages" then it might be possible.

[ponyisi]

So I'm considering a hybrid system here, where within the servicex frontend we directly import the classes into servicex.query and then the plugin system only works for external packages. Maybe not optimal but would work I think.