Bulk create, update, delete abilities for the rules client

[FrankHassanabad]

Describe the feature:
Bulk rule management functions on the Alerting Framework RuleClient, including bulkGet, bulkCreate, etc. The bulk functions should share authorization logic across alerts to reduce the CPU load compared to repeated calls to the non-bulk analogous function.

Proposed bulkGet implementation: https://github.com/marshallmain/kibana/blob/rules-status-aggs/x-pack/plugins/alerting/server/alerts_client/alerts_client.ts#L386-L433

Specific use case:
On the detection engine rules management page we display the status of each rule and refresh the statuses periodically. Our rule status API currently calls AlertsClient.get for each rule on the page to retrieve the alerting framework status for the rule. Even though we use async to make the calls to AlertsClient.get, there is significant CPU cost in the authorization logic of get and the CPU usage ends up becoming the dominant factor when fetching hundreds of alert statuses. A bulk function that can fetch multiple alerts while sharing the same authorization would allow our status route to be much faster.

Other bulk routes would be useful as well. bulkCreate in particular would have immediate applications for the detection engine, as we have a bulk create route that makes repeated calls to the AlertsClient.create function.

Original description

**Describe the feature:** Bulk create, read, update, delete for alert client

Describe a specific use case for the feature:
Users with signals are doing bulk actions such as initializing pre-defined rules on the server and right now we do manual looping over the alert client to do these actions such as create, update, and delete.

We use find for bulk reads at the moment.

[mikecote]

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

[pmuellr]

We've also had requests for bulk fetching of alert state and alert status (not yet merged, but returns data from the event log).

As an example, see issue #70169 - the data for the pie chart at the top could be created from a bulk alert state request.

[FrankHassanabad]

"Reviewed by Frank Hassanabad on 7/29/2020, still valid as of this date"

[XavierM]

we are doing bulk update with this ticket -> #124715