Add sample for how to AuthN with B2C and Github

[c-w]

This pull request contains a sample of how to authenticate with Github via AAD B2C and Azure Functions EasyAuth which corresponds to the part of the architecture highlighted in red below:

The authentication flow works like this:

1. User goes to Functions app.
2. EasyAuth redirects to AAD B2C.
3. AAD B2C redirects to Github.
4. User logs in.
5. Github redirects to AAD B2C.
6. AAD B2C redirects to EasyAuth.
7. Functions app receives AAD B2C id token.
8. Functions app uses id token to request Github access token from AAD B2C.
9. Functions app makes request to Github API via access token.

If you want to test the flow, you can use my test endpoint.

[alexgolesh]

Looks good. Thanks for putting together the sample and flow!