Clarify async credential requirement and update examples

[nateprewitt]

adlfs uses the async Azure SDK under the hood, which requires async credentials. A common source of issues is users passing synchronous credentials (e.g. azure.identity.DefaultAzureCredential) instead of their async counterparts (azure.identity.aio.DefaultAzureCredential). This was partly due to inaccurate documentation in the project which should now be fixed.

For almost all users, the preferred option is to use anon=False (soon to be default) when instantiating your file system. This allows the SDK to auto-detect the right credentials. If you must supply credentials directly, make sure you're using the async version.

This PR:

• Emits a FutureWarning when synchronous credentials are detected
• Adds a CredentialType alias for accurate type checking
• Updates docs and README to recommend anon=False over passing credential objects directly, and corrects references to point to azure.identity.aio

[anjaliratnam-msft]

This looks good to me!