Updated mock service endpoint in MockProvider

[shweaver-MSFT]

Fixes #140

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

MockProvider is currently broken because the underlying service endpoint changed.

What is the new behavior?

Instead of pointing to a hardcoded proxy url, we now make a request to a middle service to get the proxy url, and then use that for any outgoing requests.

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] Tested code with current supported SDKs
• [ ] Sample in sample app has been added / updated (for bug fixes / features)
  • [ ] Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• [x] Tests for the changes have been added (for bug fixes / features) (if applicable)
• [ ] Header has been added to all new source files (run build/UpdateHeaders.bat)
• [x] Contains NO breaking changes

Other information

[ghost]

Thanks shweaver-MSFT for opening a Pull Request! The reviewers will test the PR and highlight if there is any merge conflict or changes required. If the PR is approved we will proceed to merge the pull request 🙌