* Support SASL mechanism EXTERNAL in Erlang AMQP 1.0 client
* Move test to plugin rabbitmq_auth_mechanism_ssl
In theory, there can be other plugin that offer SASL mechanism EXTERNAL.
Therefore, instead of adding a test dependency from app rabbit to app
rabbitmq_auth_mechanism_ssl, it's better to test this plugin specific
functionality directly in the plugin itself.
This is the recommended way with Erlang.mk.
By default, the version is inherited from rabbitmq-server-release when
the source archive is created, or computed from git-describe(1) (see
`rabbitmq-components.mk`). One can override the version from the command
line by setting the `PROJECT_VERSION` variable.
[#130992027]