Support headers for fallback registries

[ting-hong-shieh]

Closes #106.

Summary

• Add headers for non-sensitive fallback registry headers and headers_env for a JSON object stored in a Worker secret.
• Merge secret headers over static configuration, using case-insensitive header names.
• Send configured headers to the registry and same-origin authentication endpoints while removing them from cross-origin token and redirect requests.
• Let generated Basic or Bearer credentials override a configured Authorization header.
• Document the configuration and Cloudflare Access service-token example.

Testing

• pnpm test (63 tests)
• pnpm run typecheck
• pnpm run lint
• pnpm run format:check