Etsy provider (WIP)

[codemasher]

Types of changes

What types of changes does your code introduce?

• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] New OAuth provider
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation fix or enhancement (no code was touched)
• [ ] Other (CI, dependencies, etc., please describe)

Checklist:

• [ ] I have checked to ensure there aren't other open Issues or Pull Requests for the same update/change
• [x] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation (if appropriate)
• [ ] Any dependent changes have been merged and published in downstream modules
• [x] Static analysis and unit tests pass locally with my changes

Further comments

API access pending, cannot verify functionality yet.