Bug 1702206: Refactor Uploader into class for external use

[ChinYing-Li]

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 && npm run lint:circular-deps 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

This PR aims to tackle bug 1702206; no new test added but some test cases have been modified.
Please ignore my question on Bugzilla, as I now understand why this approach is preferred. PR ready for review, and will modify CHANGELOG.md if there's no problem. Thanks!

[brizental]

Thanks for the work here @ChinYing-Li ! This looks great. I left some very minor comments, but otherwise this is good to go.

[brizental]

Thanks for working on this @ChinYing-Li ! Looks great :D Would you mind rebasing it so I can merge?