Implementation of multi-arch support for extension packages

[pacostas]

Summary

This implementation is based on the RFC https://github.com/buildpacks/rfcs/blob/main/text/0128-multiarch-builders-and-package.md

This PR allows the pack extension package to generate multi-platform OCI images and create an image index to combine them.

It also adds the --path flag to point to a directory on the extension that needs to be packaged.

This PR has not been tested in packaging a builder with multi-platform extension.

Output

Before

• We were not able to create multi-platform extensions
• We were not able to point to the directory of the extension

After

• Can generate multi-platform OCI images by using the --target flag
• Can specify the path of where the extension is located, using the --path flag.

Documentation

• Should this change be documented?
  • Yes
  • No

Related

Resolves

• Add --path flag to pack extension package #2183
• pack extension package should be able to create multi-arch images for extensions #2160