Add support for Base64 encoded auth config

What does this MR do?

this merge request adds a base64 decoding preprocessing for auth_password and request_headers via new environment variables and command line options.

Why?

secrets for dast on-demand scans are base64 encoded before being saved in the monolith. this is done in order to allow the variables to be masked by default in the runner output. as a result, dast may receive two new environment variables DAST_PASSWORD_BASE64 and DAST_REQUEST_HEADERS_BASE64.

Related Issue(s)

• gitlab-org/gitlab#225406 (closed)

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer