Backend: Adjust the create pipeline service workflow to work with inputs

Summary

As a part of Add support for spec:inputs in GitLab CI pipelines (&16321 - closed); adjust the create pipeline service workflow to work with inputs. (scope: only service adjustment, not endpoints)

You can see some "hacky" ideas from the POC MR;

  • Ci::CreatePipelineService needs to accept inputs as an execute parameter.
  • Pass to the pipeline config and expand it accordingly.

Validation; as we also discussed in #519950 (closed), we need to decide how to enable this feature. For example;

  • If restrict_user_defined_variables is enabled, then allow using inputs.
  • If restrict_user_defined_variables is disabled, then disallow using inputs.
Edited by Furkan Ayhan