Bug 1990965 - Add an installer attribute for the /LaunchedBy stub installer argument

[dmcintosh-moz]

This adds a launched_by attribute to the installer schema to match the work in mozilla/gcp-ingestion#2841; see there for more details. Overall project is tracked in FIDEDI-1338 ≈ bug 1990965.

Not sure whether there's an ordering in templates/*, or whether the desktop launcher should be mentioned in the description.

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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