Close #12267: Add support for Save to PDF in GeckoEngineSession

[jonalmeida]

Adds support for Save to PDF from the GeckoSession by plugging the API into onExternalResponse to provide the same flow as a typical file download experience would be.

Co-authored-by: Olivia Hall

TODO:

• [x] Add tests after first pass of draft from reviewers.
• [x] Add changelog before merge.

Pull Request checklist

• [x] Quality: This PR builds and passes detekt/ktlint checks (A pre-push hook is recommended)
• [x] Tests: This PR includes thorough tests or an explanation of why it does not
• [x] Changelog: This PR includes a changelog entry or does not need one
• [x] Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features

After merge

• Milestone: Make sure issues closed by this pull request are added to the milestone of the version currently in development.
• Breaking Changes: If this is a breaking change, please push a draft PR on Reference Browser to address the breaking issues.

Fixes #12267 Fixes #12267 Fixes #12267

[mergify[bot]]

This pull request has conflicts when rebasing. Could you fix it @jonalmeida? 🙏

[jonalmeida]

@Amejia481 @ohall-m updated the patch based on the review comments and added the appropriate tests.

[Amejia481]

Thanks! I'll do some manual testing today :)