feat(inspect): implement python-native skopeo inspect

[whatsacomputertho]

In this PR, I begin to implement a python-native equivalent of skopeo inspect. I develop the following changes in support of this:

• The ContainerImageInspect class, which represents the output of skopeo inspect
• The inspectschema.py source file, with the JSON Schema for validating a ContainerImageInspect dict
• In the ContainerImagePlatform class, a new static method for getting the host platform
  • With environment variable overrides for the os (HOST_OS) and arch (HOST_ARCH)
• In the ContainerImage class, new methods for
  • Fetching the ContainerImageConfig for the image
  • Given a manifest, getting the host platform manifest if it is a manifest list, else raising an exception
  • Using the ContainerImageConfig and host platform manifest to construct and return a ContainerImageInspect instance

I finally write a new example script examples/image-inspect.py to quickly validate this new functionality against a test image. I am on darwin/amd64 so to run the example I just simply run the following from the root directory of this repository:

HOST_OS=linux python3.9 examples/image-inspect.py

For reference, see:

• Develop capability to fetch consolidated image metadata, equivalent to skopeo inspect #28

[whatsacomputertho]

FYI @cmoulliard