Support providing values for custom variables on scheduled pipelines.

Pipelines currently supports providing names of variables that are provided values when run manually for custom pipelines.

Another use case for custom pipelines is running them on a schedule.

We could allow setting a default value to provide a value for the custom pipeline (as described here: https://jira.atlassian.com/browse/BCLOUD-18771) on a schedule but if you use the same custom pipeline with two values it would require two different pipelines each with the same config but different default value.

Instead we could extend the schedule pipeline config to allow providing a value for the custom variables that we provide to the pipeline each time we run it on a schedule.