Filter on an Iteration field

Right now we can only filter the items we want to update on a project by looking at certain field types. Specifically:
- Text
- Number
- Date

I want to add the ability to filter an item by checking of its iteration fields. For example if an item in my project has field "Iteration" which can be a `Iteration 1`, `Iteration 2`, etc., I want to be able to filter based on the "Iteration" of choice.

Start by inspecting the existing filters for Text, Number, and Date fields and the GitHub Projects item-field handling. Add iteration-field filtering so items can be matched against a selected iteration such as Iteration 1 or Iteration 2, then verify that only matching project items are updated.

Written by the indexing model from the issue text.