Alerting API - provide bulk versions of create/update/get APIs

[peterschretlen]

Describe the feature:

Alerting API today works on one alert at a time. This can create performance and resource problems when trying to operate on 100s or 1000s of alerts. For example, in detection engine it was necessary to process alerting calls in small batches in parallel. Instead, the API should be able to handle bulk requests.

Describe a specific use case for the feature:

• bulk CRUD operations, such as exporting detection engine rules (bulk get), importing rules (bulk create), or enabling/disabling (bulk update)

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[peterschretlen]

Duplicate of #53144, closing