Fix: Refactor auth handling to address issue #1594

[GabageFW]

This PR refactors the authentication handling logic in the HTTPie CLI tool to fix issues outlined in #1594. Specifically, it ensures that the authentication plugin is correctly applied based on the specified auth type.

Modifications:

• Refactored the _process_auth method to handle plugins more flexibly.
• Ensured compatibility with additional authentication methods.
• Improved error handling when no authentication is provided.

Closes #1594.