Support CSP headers generation for static content on Cloudflare

[castarco]

To improve the usability of Astro-Shield, it would be good to support the generation of headers metadata specific for the Cloudflare provider, so we can generate CSP headers for static content.

Reference

• See public/_headers file
• See https://docs.astro.build/en/guides/integrations-guide/cloudflare/#headers

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