Add group key to AlertGroup and groups API

[Spaceman1701]

This change adds the group key back to the AlertGroup struct so that it's available in the groups API endpoint. I started a discussion about this with #3446.

For the most part I think this is pretty straightforward. The one bit which might need a little more work is the use of AlertGroup in the UI. Since it uses a type generated from the OpenAPI spec, the local construction of groups in the UI requires a dummy value for the key. I considered making the value optional in the OpenAPI spec, but that wouldn't be true to the behavior of the API.