PEP reserved stage validation

Currently, we don't fail the pipeline when users try to define reserved stages in other places. Instead, we create a pipeline with the reserved stages in the correct order - .pipeline-policy-pre in the beginning, .pipeline-policy-pre at the end, even if user specifies something like stages: [build, .pipeline-policy-pre, .pipeline-policy-post, test].

The following discussion from !178199 (merged) should be addressed:

• @furkanayhan started a discussion: (+4 comments)

  We should not be able to define a stage before .pipeline-policy-pre 🤔