refactor(export-campaign): extract payload types

[ajohn25]

Description

This formalizes the export-campaign task as a "chunk task" - a task that operates on chunks of campaign contacts in series. This is done by defining common interfaces that export-campaign and other "chunk tasks" should extend going forward

Motivation and Context

Setup for tackling politics-rewired#1681 - the new second pass task will also be a "chunk task"

How Has This Been Tested?

This has been tested locally

Screenshots (if appropriate):

Documentation Changes

Checklist:

• My change requires a change to the documentation.
• I have included updates for the documentation accordingly.