Allow login screen for preflight without basic auth

[todvora]

/nocl

Description

To allow nice welcome page with login form for preflight, we have to exclude a login url from basic auth filter. This PR implements backend part of #18910. The /login url is now excluded from the basic auth.

Motivation and Context

#18910

How Has This Been Tested?

Added unit tests

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.