(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.
I think if for line 742 you use [invalid-fields (remove (set allowed-fields) (ks/as-collection field))] you can handle both the vector and non-vector cases in a single "when-not empty" clause, using some conditioning on count > 1 to decide on the plurality of "field". This would allow you to delete the second half of the function, which is mostly duplicate, and also handle the single-element vector correctly. If you go this route you may want str/join rather than (json/generate-string invalid-fields), to make it neutral between vector and string input.

Also, more of a nitpick but I'd prefer if the error-contex variable here were called error-context like it is in the description.