Key fields on 'Campaign recipients' endpoint

[dnlmc]

Currently, key fields on the Campaign recipients endpoint are designated as:

KEY_PROPERTIES = ['CampaignID', 'EmailAddress']

But I wonder if 'ListID' should be added to the key fields, since in theory a campaign can be sent to multiple lists, and the same email address might be present on those lists (& that'd be something we'd want to know). Also, in this scenario, we'd just lose one or many valid listIDs if the keys were deduplicating properly (Issue 3).