(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.
If the intent here is to check whether a field is a simple string field or a path of field components, the `vec?` check seems a little too specific, since there's no hard guarantee that the field will be a vector instead of a list. I think `sequential?` is a better predicate, since it'll return true given any sequential (i.e. ordered) collection (e.g. a vector, a list, or a lazy sequence).