Authentication with credential acquired in cloud shell fails

[wpbrown]

Describe the bug
Trying to use a credential retrieved from the CLI fails when using Cloud Shell MSI because the token format does not match the code.

To Reproduce

from azure.common.credentials import get_azure_cli_credentials
credential, _ = get_azure_cli_credentials(resource="https://storage.azure.com/")

Use the credential in the blob client...

  File "/.../site-packages/azure/core/pipeline/policies/_authentication.py", line 93, in on_request
    self._token = self._credential.get_token(*self._scopes)
  File "/.../site-packages/azure/cli/core/adal_authentication.py", line 64, in get_token
    return AccessToken(token, int(full_token['expiresIn'] + time.time()))
KeyError: 'expiresIn'

Expected behavior
adal_authentication.get_token returns a valid AccessToken.

Environment summary
azure_cli_core-2.8.0 in Azure Cloud Shell.

Additional context

The actual full_token:

{
    "access_token": "{REMOVED}",
    "refresh_token": "",
    "expires_in": "3900",
    "expires_on": "1593791553",
    "not_before": "1593787653",
    "resource": "https://storage.azure.com/",
    "token_type": "Bearer"
}

The code looks for expiresIn, but under cloud shell the full_token contains expires_in and expires_on.

[yonzhan]

add to S173