(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 mentioned conditioning on count > 1 inside this when, it was to decide on whether this should be "field" or "fields" in the error.  Since in most cases people will probably be passing a string rather than a vector, using "fields" for both cases will look like a typo most of the time. Previously it seemed like that was the reason for splitting into two cases. You could handle this by parameterizing the word in the let statement or having an if in the body of the format call.