Refactor common logic out of ImageBuilder into a shared library.

[MichaelSimons]

There is becoming a fair amount of logic defined within the ImageBuilder tool that could be shared elsewhere. Examples include: FilePusher, Docker repo unit tests (DockerHelper).

This shared logic could easily be factored into a dll that is easily shared within this repo. It could be shared with dependent repos via a NuGet package.

[lbussell]

Work is currently happening to move ImageBuilder's Model classes out to the ImageBuilder.Model package. This will allow consuming repos to interact with the Manifest and ImageArtifactDetails models programatically and reduce code duplication across repos.

Here is how I see this work at the moment:

• Migrate Manifest models from ImageBuilder to ImageBuilder.Models #1943
• Migrate Image Models to ImageBuilder.Models - I'm currently iterating on this, it's more involved than the Manifest model.
• Add tests for Manifest serialization and deserialization #1899
• Add tests for Image Model serialization and deserialization
• Migrate all models from Newtonsoft.Json to System.Text.Json
• Enable nullable annotations in ImageBuilder.Models
• Convert all model classes to records with immutable collections
• Create helper services/interfaces for reading/writing/querying models from Json