Add Authentication Options for DAST API On-Demand screens

Problem

The design for DAST API on-demand scans does not support the use of authentication. Depending on the use case for customers, they may have a need to pass in username/passwords for basic authentication or use another method.

While they can put authentication values in the header fields, this is not a good practice as all sensitive information should be encrypted in the database, or use CI variables.

Proposal

• Update the Excluded paths and Additional request headers fields as follows:
  • Add documentation on the screen on how to use https://docs.gitlab.com/ee/user/application_security/dast_api/#token-doesnt-expire
  • Update the description text for both form fields
  • Move the current description text to the help text area (below input)
• Add fields for inputting username/password for basic authentication.

🎨 Figma designs

Implementation Details

• frontend: #367533 (closed)
• backend: #350514 (closed)
• documentation: TBD
• feature flag: #345837 (closed)