Add list_modes in leaves router

The configs router got a `get_all_modes` endpoint ([issue](https://github.com/AllenNeuralDynamics/ficus/issues/78)), and it has a "lowest_scope_only" argument if you only want to list modes under one specific scope. I think this would probably be better served by a list_modes endpoint in the leaves router somehow. So the two use cases would be:

1. Tell me all the available modes for my situation (including all scopes). This looks through all scopes and makes one set.
2. Specifically tell me the mode leaves in this one scope, so I know if there's overrides.