Bug 1856730 - store debug values for session

[rosahbruno]

Store the debug values set via window.Glean in sessionStorage which persists until the user closes the current tab.

I am not exactly sure what tests for this might look like. The only way to really write tests for this would be inside of BrowserStack, but to actually verify it works you need to see either console output or an entry in the debug ping viewer.

Doc updates - mozilla/glean#2638

Screen.Recording.2023-10-04.at.10.22.28.AM.mov

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

[travis79]

This looks good to me, please be sure the behavior is documented in Glean docs