Add HTTPie for API testing with comprehensive usage guide

- Add httpie package to home.packages for modern HTTP client capabilities
- Include comprehensive usage examples in README covering:
  * Basic HTTP methods (GET, POST, PUT, DELETE)
  * Authentication with Bearer tokens and API keys
  * File uploads (form and multipart)
  * JSON data handling with type coercion
  * Session management for persistent authentication
  * File downloads and response filtering

HTTPie provides a user-friendly alternative to curl for API testing and development workflows.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

flake.nix

@@ -46,6 +46,7 @@
         python312Packages.pip
         python312Packages.virtualenv
         lftp
+        httpie
         claude-code.packages.aarch64-darwin.claude-code
         nerd-fonts.fira-code
         nerd-fonts.jetbrains-mono