Create better index routes for addons releases

[ahal]

When we release addons, we add these index routes: https://firefox-ci-tc.services.mozilla.com/tasks/index/mozillavpn.v2.mozilla-vpn-client.release.v2.14.0.latest.addons/stage

But these index routes aren't great because:

1. Addons aren't tied to mozilla-vpn-client versions
2. It looks like we only track uploads to the stage dir (in theory the latest ones here should have been run on the same revision as the task that moves them over to the release dir.. but I guess in practice we could have messed this up)

I think we should instead add the following routes:

• mozillavpn.v2.mozilla-vpn-client.release.latest.addons.candidate
• mozillavpn.v2.mozilla-vpn-client.release.latest.addons.release

We could also add the latest overall release tasks to this release.latest index.. though I'm not sure if this would be useful or just add confusion, probably the latter.

┆Issue is synchronized with this Jira Task