Ensure no breaking changes in on-demand DAST scan pipelines

Problem

As we restrict pipeline variables by default we have an internal usage of pipeline variables when creating on-demand DAST scan pipelines.

As new projects in new namespaces will have pipeline variables disabled by default, on-demand DAST pipelines will likely break.

Proposal

• Option 1: Rewrite the config not to use pipeline variables but to add variables at job level.
• Option 2: Exclude ondemand_dast_validation pipeline source when enforcing pipeline variables permissions (since this is an internal generation of pipeline).