Add params to campaign memberships

[joshuap]

Given a campaign TrialConversionCampaign, I want to be able to specify a trial end date when adding users to campaigns, so that I can reference the date in campaign emails (i.e. "Your trial is ending on 2020-02-26"). These values are unique to each user in the campaign, but may not be persisted on the user object--in this case, the value is part of an event that is triggering the campaign for the user. It would be nice to be able to add/override params like this:

TrialConversionCampaign.add(user, params: {
  ending_on: "2020-02-26"
})

I think this could be accomplished by adding a params jsonb/hstore/etc. column to the heya_campaign_memberships table. A nice extra would be to transparently support Global IDs for referencing objects.