[Logs UI] [Alerting] Add "group by" functionality

[Kerry350]

Note: This ticket covers the server side (executor) and UI side.

We should add "group by" functionality to the logs alert type.

This will work in the same way as the metrics group by.

This will allow users to set some criteria to be checked per group.

Technical considerations:

• Groups may produce large results, there could be thousands of host.name as an example, so this should be a composite aggregation.

[elasticmachine]

Pinging @elastic/logs-metrics-ui (Team:logs-metrics-ui)