[Task]: Add a new `on_version_scanned` event

[willdurand]

Description

We'll need a new on_version_scanned event in the scanner pipeline.

• This event should be executed after all scanners hooked to on_version_created have returned their results
• The payload for this event should contain all the scanner results available for the version at the time of firing this event
• Scanners hooked to this event should block auto-approval

Here is how I'd document this event in the scanner pipeline docs:

This event occurs once every scanner listening to on_version_created has
returned its results for the version, whether it returned them synchronously,
asynchronously, or skipped the event. It is meant for scanners that need to make a
decision informed by what the other scanners found.

In addition to the usual addon and version properties, the payload contains a
scanner_results object mapping the name of each scanner listening to
on_version_created to the results it returned. A null value means that
scanner skipped the event.

{
  "addon": {
    // Similar to the `on_version_created` event.
  },
  "version": {
    // Similar to the `on_version_created` event.
  },
  "scanner_results": {
    "some-scanner": {
      "version": "1.0",
      "matchedRules": ["SOME_RULE"],
      "annotations": {}
    },
    "some-other-scanner": null
  },
  "event": "on_version_scanned",
  "scanner_result_url": "http://olympia.test/api/v5/scanner/results/126/"
}

Note: When no scanner is listening to on_version_created, this event is still sent,
with an empty scanner_results object.

Two things are tied to this event:

• Like during_validation and on_version_created, it blocks auto-approval:
  a version is not auto-approved until every scanner listening to it has returned
  its results. When that takes longer than WAIT_ON_SCANNERS_TIMEOUT, the version
  is flagged for human review instead.
• The scanner actions for the version are executed once this event completes, i.e.
  when the whole pipeline is done and all the results are known.

Acceptance Criteria

see above

┆Issue is synchronized with this Jira Task