Add event overlays

[cory-stripe]

Summary

Adds support for event overlays (and selected event overlays) to dashboards

Motivation

Specifying common event overlays (deploys, etc) in terraform definitions is very useful.

It looks like this:

selected_event_overlay {
    line = true
    label = "Deploy events"
    color = "emerald"
    signal = "deploy"

    source {
      property = "host_env"
      values = ["prod"]
    }

    source {
      property = "service"
      values = ["veneur-proxy-srv"]
    }
  }

Notes

• Selected overlays and normal overlays are a bit weird. To make an event overlay that is also "selected" requires you to enter it as both an eventOverlay and a selectedEventOverlay. It might be better to just put a flag in here and do that for them. I'd love some philosophic guidance on that.
• I don't think there's any testing possible here that isn't actively hitting the API, but it works for me? :)
• I chose to pluralization using filter as a guide
• I validated the color using the existing mechanisms
• This includes some logging from #28 cuz it was SUPER helpful, hoping I can remove that in a rebase
• I assume I should add docs, unsure how it works at a glance…

Thanks and let me know how I can improve this!