Add Campaign Analytics API

[proffalken]

I'm working on a prometheus exporter for Keila so we can visulise emails etc in Grafana.

The output of the API on the most recent docker container does not include any stats, so we cannot get a graph of opens/delivers/etc.

[wmnnd]

Thank you for suggesting this. I agree it would be nice to have the analytics in the API.

We should add a new Analytics API at /api/v1/campaigns/:id/stats. The endpoint should return the all data points from Keila.Mailings.get_campaign_stats/1 plus the campaign_id.

Implementing this should be fairly straightforward by adding a new controller in lib/keila_web/api/controllers/api_campaign_controller.ex, defining the view in lib/keila_web/api/views/api_campaign_view.ex. It would be good to also add a test in test/keila_web/api/api_campaign_controller_test.exs.
Keep in mind that all API controller routes are spec'ed using OpenApiSpex and the new route should follow this best practice.