Refactor DAST on-demand config generation

at the moment, dast config generation and presentation is handled in two places (params_create_service.rb and dast_scan_ci_configuration_service.rb . this differs from the approach taken elsewhere.

implementation plan

• create ci_variable methods in dast_site_profile, dast_scanner_profile, etc, that return ::Gitlab::Ci::Variables::Collection.new (!61330 (merged))