As an admin, I want the SitePolicies policy to support an EphemeralStorage field, so that I can remove all site data for the matched site when the isolated site session ends.

Acceptance criteria:

- EphemeralStorage (boolean): Clears all site data for the matched site when the isolated site session ends. 

Implementation notes:
- https://firefox-admin-docs.mozilla.org/reference/policies/sitepolicies/
- https://firefox-admin-docs.mozilla.org/reference/policies/sanitizeonshutdown/
- When the isolated session ends, the browser purges cookies, localStorage, IndexedDB, cache, etc. for that site automatically. Session-end trigger is last tab for the origin closed / overlaps with SanitizeOnShutdown.
  - one proposal was to reuse Private Browsing mode for this.
  - Optionally, consider if an admin could enforce a time-to-live for the session (e.g. auto-delete after X hours / days of inactivity)

Out of scope:
- New site-matching syntax; constraints reuse the existing SitePolicies site-matching mechanism.