`pipeline template converter` improvements

Discovered over the course of completion of #7647.

- [ ] Auto-add @self to template references in stage template files (@scbedd)
- [ ] See if we can fix ordering (so variables goes above jobs), but without having to declare inner types for everything (otherwise we could use the order attribute in yamldotnet)
- [x] Preserve multi-line powershell arguments with yaml `>` operator (@benbp)
- [x] Figure out pool refactor as separate task
- [x] Complete publish template in eng/common
- [x] Simplify publish and just use PublishPipelineArtifact for everything (remove PublishType) instead PublishBuildArtifact. Use inline replace for nuget instead of common template (@benbp)
- [ ] Identify issue with eng/common/pipelines/templates/steps/publish-artifact.yml, the tool entirely deletes the content.
- [x] Inject full necessary `sdl` configuration
- [x] ~~Modify `eng/common/pipelines/templates/steps/publish-artifact.yml` to properly handle individual files by placing said file in a separate folder before uploading that _folder_.~~
- Just doing this in steps before the artifact publish attempt.