Bug 1867821 - Implement an uploader that defaults to sendBeacon and falls back to…

[badboy]

First try at this to get it going, see inline comments for questions.

Pull Request checklist

• Quality: Make sure this PR builds and runs cleanly.
  • Inside the glean/ folder, run:
    • npm run test Runs all tests
    • npm run lint Runs all linters
• Tests: This PR includes thorough tests or an explanation of why it does not
• Changelog: This PR includes a changelog entry to CHANGELOG.md or an explanation of why it does not need one
• Documentation: This PR includes documentation changes, an explanation of why it does not need that or a follow-up bug has been filed to do that work
  • Documentation should be added to The Glean Book when making changes to Glean's user facing API
    • When changes to the Glean Book are necessary, link to the corresponding PR on the mozilla/glean repository
  • Documentation should be added to the Glean.js developer documentation when making internal code changes

[rosahbruno]

LGTM, I will file the follow up bug discussed offline and in previous comments.

[rosahbruno]

@badboy
Would you mind making a follow up PR to this with a changelog entry? We don't need to be too descriptive since I am going to create a google doc with info about the uploaders.