Enable auto-purge for artifacts in ACR

[toddysm]

What is the problem you're trying to solve
Currently ACR supports two different ways to purge images: the untagged manifests retention policy as well as the acr purge command. Customers are requesting a more comprehensive and flexible capability to purge artifacts that can act on different conditions like artifact types, artifact metadata and even last pull date.

Describe the solution you'd like
ACR PM team is working on a proposal for feature called auto-purge that will allow customers to configure rules that will be executed on a regular basis to purge artifacts from the registry.

[JXavierMSFT]

#654

Customers would like the functionality to purge based on last pulled date.