Add return type for directory skeletons

[stuartcampbell]

Use a model for the various directory return types, e.g. in the beamline_service.directory_skeleton() function (https://github.com/NSLS2/nsls2api/blob/main/src/nsls2api/services/beamline_service.py#L322)

This can then be return also by the API route(s).

We can then include in the testing.