(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.
Fair enough. The user will never be able to pass in another container type (since the JSON parsing will always produce a vector), and if internal changes pass a list or lazy sequence by accident that should cause tests to fail, so this seems fine.