[Feature] Add a FilterExpressions API

[ByronBecker]

Motivation

Developers may with to have a handy way of filtering query results on-top of CanDB's already existing scan functionality. This would allow for a developer to attempt to return only the results where an additional condition is met in an entity's attributes.

Requirements

• Provide a filtering mechanism for developers outside of using the SK or secondary indexes
• Note that this filtering mechanism would not be sorted
• Provide limits to ensure that misuse of this API does not result in performance degradation/cost hits due to inefficient queries (i.e. scanning all elements in the structure with a single call)