* 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.
Bazel build files are now maintained primarily with `bazel run
gazelle`. This will analyze and merge changes into the build files as
necessitated by certain code changes (e.g. the introduction of new
modules).
In some cases there hints to gazelle in the build files, such as `#
gazelle:erlang...` or `# keep` comments. xref checks on plugins that
depend on the cli are a good example.