Add /cache-clear authenticated endpoint

[andrewleith]

Summary | Résumé

This PR adds a new endpoint, /cache-clear that can only be called using the cache-clear secret. This endpoint will be called upon deployment to ensure our redis cache gets flushed each time.

Related Issues | Cartes liées

• As a user, I want to clear the cache after a deploy notification-planning#1612

Test instructions | Instructions pour tester la modification

• /cache-clear can be called when authenticated using a JWT using the specific secret
• /cache-clear CANNOT BE called unauthenticated
• /cache-clear CANNOT BE called when authenticated with the admin secret
• /cache-clear CANNOT BE called when authenticated with the SRE secret

Release Instructions | Instructions pour le déploiement

None.

Reviewer checklist | Liste de vérification du réviseur

• This PR does not break existing functionality.
• This PR does not violate GCNotify's privacy policies.
• This PR does not raise new security concerns. Refer to our GC Notify Risk Register document on our Google drive.
• This PR does not significantly alter performance.
• Additional required documentation resulting of these changes is covered (such as the README, setup instructions, a related ADR or the technical documentation).

⚠ If boxes cannot be checked off before merging the PR, they should be moved to the "Release Instructions" section with appropriate steps required to verify before release. For example, changes to celery code may require tests on staging to verify that performance has not been affected.

[whabanks]

LGTM!
✅ Able to hit the endpoint with the new CACHE_CLEAR_CLIENT_SECRET
✅ JWT tokens using the Admin and SRE secrets do not allow access
✅ Cache keys clear as expected