[Azure:Python] Add audience kwarg to constructor

[lmazuel]

Clear and concise description of the problem

Today, we generate client constructor that takes credential_scopes as an array, for instance:

self.credential_scopes = kwargs.pop("credential_scopes", ["https://cognitiveservices.azure.com/.default"])

For Azure only, we want to add audience, that works this way:

• It's a string parameter
• It's optional, and default to the scope of the TypeSpec, but without /.default. For instance, https://cognitiveservices.azure.com
• If credential_scopes xor audience are used, they are winning over the TypeSpec definition.
• If both credential_scopes and audience are present as user input at the same time, log a warning and use credential_scopes (ignore audience
• Document `audience as a docstring, and make it a kwarg only syntax

Checklist

• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.