Accept transfer-encoding: chunked instead of content-length when getting MSI token

When getting the MSI token in the AzureADAuthenticator class, the response gets chunked if the token size crosses more than 2048. This returns a header transfer-encoding: chunked instead of content-length. The code is written so that it only relies on content-length if available. This should be changed and transfer-encoding: chunked should also be accepted.