feat(metrics): Log `entrypoint` to the metrics.

* Add the FxDesktopRelier that imports `entrypoint` and `context` from the URL.
* Relier extended to import `service`, `redirectTo`, `client_id`, and `state`
* Initialize either the FxDesktopRelier or normal Relier on startup.
* Instead of Metrics importing values directly from the URL, they are fetched from the Relier and passed in on creation.

fixes #1568
Merge pull request #1639 from mozilla/issue-1568-desktop-entry

feat(metrics): Log `entrypoint` to the metrics.

@kparlante gives the good to go, merging.