Allow provider field descriptions to be provided in the config

[ml-evs]

Currently if you want to describe a provider field in the relevant /info/<endpoint> endpoint, you have to add it explicitly to a pydantic model.

Instead of a simple list of provider fields like provider_fields = {"structures": ["volume", "band_gap"], we should also allow something like

provider_fields = {
    "structures": [
            {"name": "volume", "description": "The cell volume", "units": "A^3"}, 
            {"name": "band_gap", "description": "The band gap", "units": "eV"}
    ]
}

s.t. the description/units etc. can be propagated to the relevant info endpoint.