(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.
The intent here is to validate query field(s) which can either be a string or a vector of string. This is described in the API documentation: https://docs.puppetlabs.com/puppetdb/master/api/query/v4/operators.html#subquery-operators

Since this function is used in query validation, I think it should only allow what is defined in the docs and fail if query fields are passed using a different container type.