Align the interval with values used in sylva-core

As noted in sylva-projects/sylva-core!4101 (comment 2750993501), we should provide a way to increase the source interval similarly to what has been done in sylva-projects/sylva-core!4366 (merged)

For that purpose, we can add an interval parameter to SylvaUnitsRelease.spec.sylvaUnitsSource to allow the tuning of this parameter.