Azure identity needs a "whoami" function.

Greetings,

**Is your feature request related to a problem? Please describe.**
Debugging Azure permissions is often very confusing, especially in least-permissions scenarios where you don't have access to the environment. I want to be sure my code is running with the correct managed identity.

**Describe the solution you'd like**
A function along the lines of this:

```python
credential = DefaultAzureCredential()
principal_details = azure.identity.whoami(credential)
logger.info('Authenticated as: %s %s', principal_details.name, principal_details.client_id)
```

This would help identify misconfiguration, especially in the unfortunate case where a different team manages the environment in which the code runs.