Add uniqueness for pipeline_execution_schedule_policy to schema

With scheduled pipeline execution policies (PEP) it is possible to enforce the start pipelines in each project that the policy applies to based on a schedule. More details in &14147.

The feature has the potential to trigger many pipelines at once if applied to a large group. In order to keep the number of pipelines as low as possible, we want to avoid duplicated policies. Therefor we can make sure pipeline_execution_schedule_policy items are unique using the JSON schema.