This is a wholesale rewrite of the way that we generate container images to support better MSBuild incrementality (as well as controlling/caching the location that the SDK stores the pulled base image layers/manifests/etc).

In general, today we re-do a lot of work for each individual image:

• manifest pulling
• RID selection
• app layer creation
• app manifest/config creation
• publishing to various destinations

We do all of these things per-RID, every build.

With this PR we attempt to:

• pull manifests once (and support caching them!)
• pull layers once
• use the data from this stage as inputs to the rest of the container generation
• make the app-level layer, configuration, and manifest generation incremental
• separate the creation of the artifacts from the sending of the artifacts
• disentangle the giant ball of mud that CreateNewImage is :)

Right now, for single-RID containers we are very incremental! The main gaps are:

• rebuilds are missing some data (need input from @rainersigwald on the best way to ensure that Items created during Task execution are actually handled correctly across runs)
• the multi-arch manifest creation/publishing isn't yet created