Expand specialist document metadata from schema

When requesting extra fields such as case_type, we want to look up the label in the schema. This commit does that by passing around the index mappings, getting the extra fields and looking them up in the relevant schema. `allowed_return_fields` now returns the array `ALLOWED_RETURN_FIELDS` and the ones in the schema.
This mutates the input. Mutating `params_to_expand` would be fine since we made it, and then we can merge together `normal_params` and the mutated `params_to_expand` to produce the result we want to return.

Though, that wouldn't retain the order of the original, so if that's important we should find a different approach. Let's chat about this when you pick it up.