Mask http header values

What does this MR do?

Masks HTTP Headers in the DAST report so that sensitive information isn't displayed. Masking will replace the real header value with a series of *.

When the DAST_MASK_HTTP_HEADERS environment variable is not supplied, the headers Authorization, Proxy-Authorization, Set-Cookie and Cookie will have their values masked.

Cookie and Set-Cookie are treated like special cases as only the values in the value will be masked, e.g. 'Set-Cookie', 'sessionId=********; Domain=site.com; Secure; HttpOnly'.

This MR has no changelog, as the feature has not been turned on.

What are the relevant issue numbers?

gitlab-org/gitlab#215679 (closed)

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