feat: Support custom regions

[mhan83]

Proposed changes

Allow user defined regions by reading a file at ~/.sauce/regions.yml that defines a list of regions, e.g.:

- name: local
  apiBaseURL: "https://local-api"
  appBaseURL: "https://local-app"
  webdriverBaseURL: "https://local-wd"

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