Add Doppler loader

[sergey-tyan]

What is the purpose of this pull request?

Fixes https://github.com/palkan/anyway_config/issues/107

What changes did you make? (overview)

Added doppler loader

Is there anything you'd like reviewers to focus on?

Checklist

• [x] I've added tests for this change
• [x] I've added a Changelog entry
• [x] I've updated a documentation

[sergey-tyan]

No problems! Thanks for reviewing