admin/message logs: Add field for querying statistics on outgoing messages

[mastercactapus]

Description:
This PR adds a new stats field to the MessageLogsConnection type that will generate time-series data server-side.

Which issue(s) this PR fixes:
Backend component of #2481 (original issue: #2460)

Describe any introduced API changes:
The MessageLogConnection type got a new stats field containing timeSeries the new types for time-series are generic so that this pattern can be applied to other paginated queries as needed.

Additional Info:
Example query:

query {
  messageLogs {
    stats {
      timeSeries(input:{bucketDuration:"P1D"}) {
        count
        start
        end
      }
    }
  }
}