Permit customers to pass a file containing hostnames to be excluded or ignored from DAST

Proposal

For DAST_BROWSER_EXCLUDED_HOSTS and DAST_BROWSER_IGNORED_HOSTS, customers can submit a list of strings.

With the large number of hosts that may be linked to from a site, it becomes inefficient to maintain all of this in a CI variable.

💡 Proposal: permit customers to populate DAST_BROWSER_EXCLUDED_HOSTS and DAST_BROWSER_IGNORED_HOSTS by supplying a file. This could work like Use DAST_PATHS_FILE CI/CD variable.

See gitlab-com/support/support-pairing#11107.