feat(js_server): Add support for the server-deletion-request ping (DENG-4079)

[akkomar]

This adds a server-deletion-request ping so server applications can ask the pipeline to delete a user's collected data.

Applications will opt in by adding server-deletion-request to send_in_pings on the metric that identifies the user. No template changes were needed because we already had support for pings carrying only string metrics.

This doesn't reuse deletion-request ping because server telemetry has no Glean-managed client_id that's used by generic Shredder machinery.

User-facing documentation will land in mozilla/glean#3613.

See https://mozilla-hub.atlassian.net/browse/DENG-4079.

Pull Request checklist

• Quality: This PR builds and tests run cleanly
  • make test runs without emitting any warnings
  • make lint runs without emitting any errors
• 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
  • Any breaking changes to language binding APIs are noted explicitly