Add EXTERNAL as rabbit login method

As explained in the link below kombu has login method called external
https://docs.celeryq.dev/projects/kombu/en/latest/_modules/kombu/connection.html

The login method external is not listed as a choice in the Rabbit driver

As explained in RabbitMQ documention
https://www.rabbitmq.com/access-control.html
for Authentication using Client TLS (x.509) Certificate Data
clients must be configured to use the EXTERNAL mechanism.

Closes-Bug: #1970276
Change-Id: I5c38d3a3cafd49f8abc031e36bc595f32a8631d2