feat: Support regional credentials

[mhan83]

Proposed changes

Allow defining credentials for both built-in and custom defined regions.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• I have updated the json schema (if appropriate)

Further comments

• All sauce regions will still default to env vars and ~/.sauce/credentials.yml by default but can be overridden in regions.yml like so:

- name: us-west-1
  credentials:
    username: customUser
    accessKey: customKey

• Since even sauce regions can be overridden, there is no longer global default credentials. So clients now access credentials from the configured region instead of the credentials package.

DEVX-2637