Support for windows buildpackages

[micahyoung]

Requesting support for generating Windows-formatted buildpackages, which are the same as typical buildpackages, except that they are Windows-formatted OCI images instead of Linux-formatted OCI images.

They are generated with identical pack package-buildpack syntax except the package.toml contains a platform section [pack docs]:

Example:

[platform]
  os = "windows"

Additionally, pack currently requires experimental = true to be set in the ~/.pack/config.toml to create Windows buildpackages.

The pipeline-descriptor.yml section to enable this could be:

package:
  repository:   gcr.io/paketo-buildpacks/sample-buildpack
  platform:     { os: "windows" }

When the descriptor's platform section is omitted, the corresponding package.toml's platform section would be omitted (implicitly using pack's default which is currently linux)