Relax the restrictions on the pipeline template to permit the inclusion of stages.

[VonnyJap]

What happened:
Currently, the data-schema of the pipeline template does not include stages.

What you expected to happen:
Update the configuration to permit the inclusion of stages in the pipeline template.

How to reproduce it:
Attempting to create a pipeline template with stages in the configuration results in an error.

[github-actions]

Similar issues

1. 54% Pipeline template detail page missing annotations and parameters info #3091
2. 54% Remove associated stages and the builds for the stages when a pipeline is deleted #3025
3. 51% Enable customization of annotations configuration within user-defined pipelines #3080
4. 51% Enable customization of parameters configuration within user-defined pipelines #3077
5. 50% Unable to delete a specific version of a pipeline template #3072
6. 49% Enable customization of subscribe configuration within user-defined pipelines #3076
7. 48% Enable customization of cache configuration within user-defined pipelines #3075
8. 45% Users want to know that the pipeline is executed by pipeline token #3087
9. 45% Skip execution of implicit jobs created for a Stage #3027
10. 42% Adjust the border of stage in the workflow graph based on the status #3064
11. 41% Ability to Restart Pipeline for PR build when Path exists #3011
12. 39% Allow a stage to be declared as an upstream for another stage #3045
13. 38% Workflow graph - Non stage jobs overlap into the stage in the UI #3098
14. 38% Config parser changes to support Pipeline template Implementation #3020
15. 37% Update SD Guide to have information about the Pipeline Template #3037
16. 36% Validator to provide warning message when job in a stage is build periodically #3101
17. 35% Create API functional tests for pipeline stages #3042
18. 33% Support the workflow graph with stages configuration in the screwdriver UI validator #3041
19. 31% Backend change to support marking pipeline template as trusted #3056
20. 30% Add support for cache settings in pipeline templates #3058
21. 30% Remote-join job doesn't wait on downstream jobs during restart #2981

_{🤖 By issues-similarity-analysis}