Advanced scenarios using Refit

[springcomp]

Summary

This PR builds upon the Calling third-party REST APIs lesson and introduces some advanced scenarios using Refit. Specifically, this PR walks the reader through using Refit for mocking and authenticating API calls.

Those concepts were illustrated during the corresponding lesson’s live stream recording.

PR Checklist

• [x] Prerequisites README.md is complete.
• [x] Lesson README.md is complete.
• [x] Source code is complete (one project with several exercises).
• [x] Lesson Workspace is working (check Win ✔+macOS ⬜).
• [x] Link to the lesson is added to tables in Root & Lessons READMEs.
• [x] A CodeTour is available for each exercise in the lesson.
• [x] The CodeTours are working in the lesson Workspace (check Win ✔+macOS ⬜).
• [x] A Build step is added to specific GitHub workflow (if applicable).
• n/a GitHub issue is linked. Fixes: #ISSUENR.

[springcomp]

@marcduiker I finally got the chance to draft the long promised lesson for teaching mocking and authentication using Refit. Please, would you be so kind to provide feedback on this draft PR ?

CodeTour files, workspace and all bells and whistles are coming soon 😁.