Need to add custom schema entries for unit/sortable (and eventually type)

[ml-evs]

At the moment, we only hack the entry info schemas to display e.g. sortable and unit fields for entry endpoints. Instead, we should probably create a customised BaseModel class that add these keys to every Field (see the pydantic docs).

This would allow the backend to behave differently for custom schema (e.g. disallowing sorting on fields that have sorting False) and will also make it much easier to leverage our pydantic models to populate these fields.

I'm having a play with this already for #276 and #277.

[ml-evs]

Nevermind, it seem this is really easy just adding extra keys to Field directly, we just currently throw some of them away. We may need to take this approach for type if we want to use the type info from pydantic (and add some conversion to the standard optimade types).