(PDB-722) Added missing validation for fields in vector forms for 'in' and 'extract' operators in API v4

Previously such validation was only implemented for if field is passed as a string.
So, I added a validation for both when it is passed as a string and when as a vector of strings.

Original problem described in PDB-722 seems to be already fixed, so that was the only related issue that we found.
When I looked in the documentation, it says [<"FIELDS">]. So I think the error should be pretty clear to a user as he/she already knows that this argument can be array.

Anyway, this is not a big change, I can fix it.