fix azure servicebus using managed identity support

[marnikow]

When using managed identities with azure service bus the connection fails as mentioned in #1345

This PR is a continuation of #1641

[auvipy]

the tests are looking good. can you also check the lint issues please?

[marnikow]

When trying the new version I get the following error:

TypeError("argument of type 'DefaultAzureCredential' is not iterable")
Traceback (most recent call last):
  File "/usr/local/lib/python3.8/dist-packages/celery/worker/worker.py", line 202, in start
    self.blueprint.start(self)
  File "/usr/local/lib/python3.8/dist-packages/celery/bootsteps.py", line 112, in start
    self.on_start()
  File "/usr/local/lib/python3.8/dist-packages/celery/apps/worker.py", line 135, in on_start
    self.emit_banner()
  File "/usr/local/lib/python3.8/dist-packages/celery/apps/worker.py", line 169, in emit_banner
    ' \n', self.startup_info(artlines=not use_image))),
  File "/usr/local/lib/python3.8/dist-packages/celery/apps/worker.py", line 230, in startup_info
    conninfo=self.app.connection().as_uri(),
  File "/usr/local/lib/python3.8/dist-packages/kombu/connection.py", line 715, in as_uri
    return self.transport.as_uri(
  File "/usr/local/lib/python3.8/dist-packages/kombu/transport/azureservicebus.py", line 484, in as_uri
    if ":" in credential:
TypeError: argument of type 'DefaultAzureCredential' is not iterable

I don't understand the meaning of the function as_uri so I just fixed the error locally in #1825 . Feedback is welcome :)