This PR migrates the vast majority of Alerting legacy code to the Kibana Platform.
This includes:
1. Removed legacy Task Manager
2. Migrates Fixture plugins in Alerting, Triggers UI and Task Manager Perf
This does not includes:
1. The PagerDuty simulator due to a lack of support for custom responses in the platform. issue opened. https://github.com/elastic/kibana/issues/65045
2. The Webhooks simulator due to a lack of support for custom authorisation. Requires investigation.
Introduces a disabled (enabled manually when needed) Integration Test for stress testing Task Manager.
This is used to generate baseline stats for Task manager performance which can then be rerun to compare performance oriented changes and measure their performance.
