Bug 1910462 - Add msstoresignedin to legacy telemetry environment

[BenWu]

https://bugzilla.mozilla.org/show_bug.cgi?id=1910462

This was added to firefox two years ago but never added to the schema and didn't have significant volume until now https://searchfox.org/mozilla-central/source/toolkit/components/telemetry/docs/data/environment.rst#88

Verified the path with

SELECT
  additional_properties
FROM
  `moz-fx-data-shared-prod.telemetry_stable.event_v4`
WHERE
  DATE(submission_timestamp) = '2024-07-29'
  AND additional_properties LIKE '%"msstoresignedin"%'
  AND sample_id = 5

Checklist for reviewer:

• Commits should reference a bug or github issue, if relevant (if a bug is referenced, the pull request should include the bug number in the title)
• If adding a new field, the field should have a description (see Bug 1652842 - Add a blocklist field so the environment schema works with pre- and post-bug-1647225 data #576 for an example)
• If coming from a fork, run integration tests: ./.github/push-to-trigger-integration <username>:<branchname>

For glean changes:

• Update templates/include/glean/CHANGELOG.md

For modifications to schemas in restricted namespaces (see CODEOWNERS):

• Follow the change control procedure