Added code sample for ManagedIdentityCredential

[tniek]

Found the sample section not very clear, as far as other credentials are concerned; therefore, I've added sample code we're using for our product.

[chlowell]

Hi @tniek, thanks for contributing! I'm reluctant to add a sample for user-assigned managed identity because it's more advanced than what we typically foreground in the readme. In fact, we intend to revise the readme to sharpen its focus on basic getting started scenarios. Still, I can see the reference docs could be clearer about user-assigned managed identities, so I've noted that in an issue tracking doc revisions (#10734).

[chlowell]

Thanks again for contributing this pull request. I'm closing it because the scenario doesn't fit in to the readme. We'll clarify user assigned managed identity in the next revision of our documentation.