feat(amazon): Refactor Amazon metadata provider

Refactor the Amazon metadata provider, setting up a framework for refactoring
other metadata providers with the aim of making it easier to add new providers
and make providers regional.

In this PR:

- Anything expected to be common across many metadata providers is moved into
  the base Metadata class
- AmazonJp is merged into Amazon
- A new user and global setting for the Amazon region is added, enabling many
  different regional Amazon sites in one provider class
- Tests for the Amazon metadata provider are added, including testing different
  regions being set
  - There are more than just superficial differences with AmazonJp, those are
    specifically tested

Author: jgoguen