[4.x] Ability to disable CP authentication

[duncanmcclean]

This pull request adds a new configuration option, allowing developers to disable CP authentication.

Use case

This may come in helpful for sites where you already have login implemented (via Nova, Jetstream, other means) and you don't want Statamic to have it's own login / password reset pages.

[what-the-diff]

PR Summary

• Added configuration to disable Control Panel authentication routes
  The PR introduces an option to disable the Control Panel authentication routes, allowing improved control and customization.

• Updated AuthenticationException handling for disabled routes
  The AuthenticationException::toResponse() method has been updated to handle disabled authentication routes correctly, ensuring smooth functionality.

• Fixed and improved LoginFormTest for disabled auth routes
  The LoginFormTest has been fixed to work with disabled authentication routes, and additionally, a new test has been added to ensure proper behavior in such scenarios.