Add support for campaign metrics

[mycotics]

Summary

A campaign metrics stream would be very important for analytics.
There is an api endpoint for it, but we probably need to query the active campaigns first
https://api.iterable.com/api/docs#campaigns_metrics

Fivetran seems to support it

[ReubenFrankel]

Yep, seems doable. Would probably be quite similar in setup to experiment_metrics, with some extra filtering on campaignState for "active" campaigns, as you pointed out (although not sure if this is a strict requirement as I have not tested this API endpoint).

PRs welcome!