[feat:extensions] add support to package web-bot-auth

[archandatta]

Note

Introduces end-to-end tooling to prepare the Cloudflare web-bot-auth extension for Kernel use.

• New extensions build-web-bot-auth command with flags: --to, --url, --key, --upload, --name
• Implements build flow: download GitHub archive, extract, inject host URLs into templates, convert Ed25519 JWK to PEM, run npm install/build, bundle, and copy artifacts (.crx, update.xml, built files, policy directory, private_key.pem with .gitignore)
• Uses readable extension path names (e.g., extensions/<name>/update.xml) instead of Chrome ID in generated URLs
• Adds pkg/extensions/webbotauth.go (core logic) and tests for download/extract
• Adds utility functions in pkg/util: file ops (CopyFile, CopyDir, ModifyFile) and crypto (JWKToPEM, PKCS#8 marshal)
• Wires command into cmd/extensions.go alongside existing list/download/upload commands

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