Add a limitation when purge untagged repository

**What is the problem you're trying to solve**

The purge task will timeout if the repository's manifests number high.
Because purge command will list all the manifests before deleting them.
In our case some repo has more than 140k manifests, and the purge task get stuck and not receive the response from ACR utils even when task killed of excceeding 6-hour timeout.

**Describe the solution you'd like**
Add a `--untag-limit` argument in purge command. It can limit the amount of untagged manifest for one time