Add support to cast a user token or service token as an Azure TokenCredential type

[MattB-msft]

Many Azure provided library's provide support for accepting a TokenCredential type.
In the .net and JS SDK's we have implemented a wrapper class that will cast a access token ( user or service account ) as an TokenCredential Type.

JS: https://github.com/microsoft/Agents-for-js/blob/2aff2ef62c478c25c61c6e62c8886a5cf99fd797/packages/agents-hosting/src/auth/msal/msalTokenCredential.ts#L12

This helper needs to be added to Python.

[rodrigobr-msft]

The PR linked adds an implementation of AsyncTokenCredential. Using a synchronous TokenCredential is less performant, and AsyncTokenCredential is often accepted wherever a TokenCredential is accepted