TODO: merge with FilterOptionsFromRequest.

Filter is a collection of FilterItem, with a link operator.

FilterItem is an individual filter consisting of a field, operator, value and a not boolean that negates the operator.

Filterable interface is for anything that can be filtered, it needs to support querying the type and value of fields.

LinkOperator determines how to chain multiple filters together, 'AND' and 'OR' are supported.

Operator defines an operator used for filter items such as equals, contains, etc, as well as the arithmetic operators like ==, !=, >, etc.