feat: cache status per content

The exporter exposes cache disposition per host via cloudflare_zone_hostname_cache_status{zone, host, cache_status}, and request volume per content type via cloudflare_zone_requests_content_type_total{zone, content_type} — but no metric crosses the two. Because they share no common label, they can't be joined in PromQL either.

Add a new metric which shows the cache_status per content_type per selected host.
cloudflare_zone_hostname_content_cache_status{zone, host, content_type, cache_status}
Maybe add a dedicated toggle (e.g. HOST_CONTENT_CACHE_METRICS=true, default off).
Content types come back as Cloudflare's bucketed short names (json, html, javascript, octet-stream, ...), so the label set is small.

Use the existing cloudflare_zone_hostname_cache_status and cloudflare_zone_requests_content_type_total metrics as entry points, tracing where their host and content-type data are collected. Add the cloudflare_zone_hostname_content_cache_status metric with zone, host, content_type, and cache_status labels, and confirm the dedicated toggle is off by default and the metric appears when enabled.

Written by the indexing model from the issue text.