handle sessions event

[willkg]

If I run socorro for a bit, I eventually see one of these in Kent:

{
  "event_id":"ccb92246-db8a-47a8-ba2f-82e4127b0976",
  "payload":{
    "body":{
      "aggregates":[
        {
          "exited":99,
          "started":"2024-10-31T13:40:00.000000Z"
        }
      ],
      "attrs":{
        "environment":"production",
        "release":"none:unknown"
      }
    },
    "envelope_header":{},
    "header":{
      "content_type":"application/json",
      "length":132,
      "type":"sessions"
    }
  },
  "project_id":1
}

Kent captures these, but can't generate a summary. We should support these in summary.