Add initial Architectures support (and the beginnings of a "put-shared" subcommand for bringing all the SharedTags and arch-specific images together with manifest lists/indexes)

[tianon]

This is part of #2289 and the ever-increasing goal of officially supporting multiple architectures.

A few things that aren't supported yet here that we'd like to support:

• a fully-functional put-shared subcommand which actually creates manifest lists / indexes properly (taking both Architectures and SharedTags into account)
• validating expected Architectures values; the full initial list is as follows:
  • amd64
  • arm32v5
  • arm32v6
  • arm32v7
  • arm64v8
  • i386
  • ppc64le
  • s390x
  • windows-amd64
• automatically targeting the appropriate namespace for tag and push (so that put-shared can also deterministically use the same lookup table to re-join everything)