Integrate Save to PDF into GeckoSession

[jonalmeida]

With BZ-1659819 landed, we can integrate it into AC and test it out in Reference Browser while working in parallel with the Fenix team.

Our existing downloads flow is what we want to trigger when an action to save to PDF is invoked.

Corresponding Fenix issue: https://github.com/mozilla-mobile/fenix/issues/3709

┆Issue is synchronized with this Jira Task