Sample for auth code using x/oauth2

Thanks for your contributions and feedback to the samples for Go on Azure!

### This issue is a: (mark with an `x`)
```
- [ ] bug report (consider searching issues before submitting)
- [ ] feature request
- [ ] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)
```

### Minimal steps to reproduce
>

### Related errors and log messages
>

### Expected/desired behavior
>

### OS and Go version
> Windows version, Linux distribution and version, macOS version.
> Go version

Thanks again!