Server cache validation policy. Specifies when server-side cache entries are validated.
With expiration-based validation, source images are periodically checked whether they have changed. With catalog-based validation, source images are checked only after the
catalog::TimeStamp value changed.
Catalog-based validation is recommended when image catalogs are used. Expiration-based validation should be used when images are referenced directly, without the use of an image catalog.
Enum. 0 to select expiration-based validation, 1 to select catalog-based cache validation.
default::CacheValidationPolicy if not defined or if empty.