Enable Security Alerts to be added to Elastic Security via an API

[stuartMoorhouse]

Description

A SOC could have multiple security operations centres which will need to glue different services together, including services that produce Alerts – and part of that will be adding each alert to Elasticsearch programitcally. For example, via a REST API.

This is necessary to action, create or modify alerts that aren’t log-based or come from alternate sources.

[elasticsearchmachine]

Pinging @elastic/es-security (Team:Security)