feat: use the js_periodic directive to check/issue/renew certs

[zsteinkamp]

Uses the new directive js_periodic in njs >= 0.8.2 to invoke clientAutoMode(r) on regular intervals. This eliminates the need to set up a cron or use docker healthchecks.

Proposed changes

• Use js_periodic to invoke clientAutoMode() periodically.
• Clarify some instructions for variable use in the shared dict zone.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• [x] I have read the CONTRIBUTING document
• [x] If applicable, I have added tests that prove my fix is effective or that my feature works
• [x] If applicable, I have checked that any relevant tests pass after adding my changes
• [x] I have updated any relevant documentation (README.md and CHANGELOG.md)

[ivanitskiy]

Please ensure integration tests are updated and tests both triggers "periodic" and regular HTTP request. it would require updating nginx config in integration tests