At which level we offer configuration for attributes.
Values:
ATTRIBUTE_CONFIG_LEVEL_UNSPECIFIED (0):
Value used when unset. In this case, server behavior
defaults to
CATALOG_LEVEL_ATTRIBUTE_CONFIG.
PRODUCT_LEVEL_ATTRIBUTE_CONFIG (1):
At this level, we honor the attribute configurations set in
Product.attributes.
CATALOG_LEVEL_ATTRIBUTE_CONFIG (2):
At this level, we honor the attribute configurations set in
[CatalogConfig.attribute_configs][].