Move GetKnownArtifactsFromAssetManifests to arcade

We should invest in an Arcade task that produces ItemGroup items from a manifest. This is follow on work to https://github.com/dotnet/sdk/pull/47076. In the VMR orchestrator/arcade right now, 'important' added properties need to be dealt with in both PushToBuildStorage as well as in the orchestrator. The arcade manifest code already knows how to create model elements from ItemGroups. We should have the reverse that reads items back from the model implemented there, and then we can test a round trip.

_Originally posted by @mmitche in https://github.com/dotnet/sdk/pull/47076#discussion_r2052877180_