pack extension package should be able to create multi-arch images for extensions

[natalieparellano]

Description

In accordance with RFC 0128.

Proposed solution

#2086 implements the RFC for buildpacks, we just need to make sure that packaging works for extensions.

Additional context

• This feature should be documented somewhere

I guess whatever documentation we have for multi-arch buildpack packages and multi-arch builders should make reference that you can create multi-arch extensions. But, the end user experience is pretty much the same for buildpacks vs extensions.

See discussion here: #2086 (comment) for the work that needs to be done to make packaging work for extensions.

[jjbustamante]

Let's work on this after releasing 0.34.0 and merging #2086