[Logs UI Alerting] Create alert type and executor

[Zacqary]

Implement an alert type and executor for logs alerts.

#61493 is the main issue tracking this feature and will be the most up-to-date as requirements evolve, so use it as a reference for what the backend needs to do.

[elasticmachine]

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

[Kerry350]

Closing (#61493 and #62806 cover the work here).