Support more action spaces

[slinlee]

As part of the work to integrate with the webapp, I effectively hard-coded the policy server to be discrete+tuple actions.

We should confirm our policy server supports the types of action spaces described here: https://help.pathmind.com/en/articles/5149766-framing-action-spaces

• Support describing the action space in schema.yaml
• Return actions with labels ( Filed as Return actions as structured json with named keys #99 )

[maxpumperla]

@slinlee continuous actions are already supported, simply set discrete: False. But I guess you mean more generally "support all sorts of stuff". In that case, I think we can pull that off.

Remember in v1 of policy server we had more sophisticated ways of describing both inputs and outputs, and slimmed things down a bit. maybe time to increase complexity again.