Per-asset cache thresholds

[asilvas]

This would enable storage providers that can support it, to adhere to a maximum number of cached assets and either evict assets over the maximum (via LRU?) or fail if the maximum is reached (easier but not the ideal experience).

This effectively would greatly help deter abuse, be it using the service in unintended ways, or simply trying to bring down the system by forcing excessive image processing.

This feature would only function if the storage provider supported such a behavior, as it would require tracking cache counts.