Trigger flaky test runner via GitHub

[timroes]

It would be very nice if we could trigger the flaky test runner from within a PR via a command/bot directly.

My ideal scenario, would just be a comment: @elasticmachine flaky-test-runner and it automatically determines which test groups have been changed within the PR and trigger a run for that and leave a comment afterwards with the result.

Since I think it might be trigger figuring this out automatically (also what would you run when you just change a page object or service?), already being able to specify it manually would be a big win: @elasticmachine flaky-test-runner oss:ciGroup:5 and then get a comment under it.

What are the benefits why we should build this:

• Currently flaky test runs are kind of not documented under a PR properly. Sometimes users leave comments and link to the flaky test runs they triggered (which then will later on just vanish). Having a bot comment under the PR makes this significantly more transparent and saves it for posterity. Also currently you need to check yourself when the flaky test runner is done, a comment would trigger your regular GitHub notifications.
• We have external people working on Kibana, without an Elastic account (aka most of our visualization team). They currently cannot trigger that, since they can't log into the CI. That bot would allow every Contributor to use the flaky test runner, without requiring someone to trigger their test runs for them.

Related to #70733

[timroes]

cc @alexwizp

[elasticmachine]

Pinging @elastic/kibana-operations (Team:Operations)

[elasticmachine]

Pinging @elastic/kibana-qa (Team:QA)