Draft: Issue #3482279: Cache tags / hash collision

• Repurposes "cloudflarepurger.cache_tag_header_limit" to be the limit in bytes of the "Cache-Tag" response header contents
• Hashes cache tags via md5() only when they're longer than 32 characters
• Creates method regroupCacheTagsByLimit() to split cache tags into separate groups respecting the configured size limit in bytes
• Adds multiple "Cache-Tag" response headers when over the size limit
• Moves denylisted tags logic to its own method
• Add unit tests for cacheTagsToHashes() and regroupCacheTagsByLimit()

Closes #3482279