feat: add Authenticated Origin Pulls (AOP) certificate metrics

[baubuchon-cf]

What
Add new metrics from the Cloudflare AOP REST API:

• cloudflare_zone_aop_global_enabled — whether global AOP is enabled at the zone level
• cloudflare_zone_aop_enabled — whether zone-level AOP is enabled
• cloudflare_zone_aop_certificate_status — status of uploaded AOP certificates (active, pending_deployment, expired, etc.)
• cloudflare_zone_aop_certificate_expiry — certificate expiry date as a Unix timestamp

Why
There is currently no way to monitor AOP certificate status or expiry via the exporter. Enterprise customers (e.g. Workday) use the exporter for observability and want to alert on expiring or inactive AOP certificates — the same way cloudflare_zone_certificate_validation_status covers edge certs today.

API
Available via existing REST endpoints, no new permissions required — SSL and Certificates: Read is already in the recommended token:

• GET /zones/{zone_id}/settings/tls_client_auth — global AOP enabled state
• GET /zones/{zone_id}/origin_tls_client_auth/settings — zone-level AOP enabled state
• GET /zones/{zone_id}/origin_tls_client_auth — zone-level certificate status and expiry
• GET /zones/{zone_id}/origin_tls_client_auth/hostnames/certificates — per-hostname certificate status and expiry