[Azure Container Registry] Add nightly cleanup task


This is where we're at in the `azsdkengsys` ACR. We need to clean up some of the images that aren't referenced by external resources. For the test-proxy, this means we can clean up every single tag that was NOT referenced in an `eng/common` build.

For the other images in the registry, we should probably do the same. It'll be a case-by-case basis on those though. I'm not certain they would follow the same rules as test-proxy images do.

The issue names no repository file, test, or entry point. Start by locating the azsdkengsys ACR automation and the eng/common build references for test-proxy tags, then document cleanup rules for each image. Done means a nightly task removes only unreferenced tags without affecting externally referenced images.

Written by the indexing model from the issue text.