Prototype for system index upgrade using persistent tasks framework

[williamrandolph]

Choose a relatively simple feature such as security or logstash and prototype an upgrade process via an bwc integration test. This should be callable in a transport action invoked from the API stubbed out in #77522.

If we have trouble using persistent tasks, we should fall back to a custom use of cluster state.

Subtask of #67385

[elasticmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[gwbrown]

The prototype lived in #78951, which has since been productionized and merged. As such, I'm going to close this issue.