Improve site load times and SEO with one-click support for Signed Exchanges on Google Search

Introduced one-click support for Signed Exchanges (SXG) for Google Search. This feature leverages Cloudflare's existing expertise in certificate management and TLS delivery infrastructure to generate SXGs. The process involves fetching original content, adding a Digest header with Merkle Integrity Content Encoding, stripping irrelevant/sensitive headers, chaining headers and metadata for signature calculation, and packing the final binary. This enables faster content delivery from search engines and aggregators, improving user experience and SEO.