… module

[Why]
This will be used in a later commit to find the auth backend plugin that
provides a configured auth backend module.

[How]
We go through the list of available plugins, regardless if they are
enabled or not, then look up the given module in the list of modules
associated with each plugin's application.

…boot time

[Why]
If a user configures an auth backend module, but doesn't enabled the
plugin that provides it, it will get a crash and a stacktrace when
authentication is performed. The error is not helpful to understand what
the problem is.

[How]
We add a boot step that go through the configured auth backends and
query the core of RabbitMQ and the plugins. If an auth backend is
provided by a plugin, the plugin must be enabled to consider the auth
backend to be valid.

In the end, at least one auth backend must be valid, otherwise the boot
is aborted.

If only some of the configured auth backends were filtered out, but
there are still some valid auth backends, we store the filtered list in
the application environment variable so that
authentication/authorization doesn't try to use them later.

We also report invalid auth backends in the logs:

* Info message for a single invalid auth backend:

    [info] <0.213.0> The `rabbit_auth_backend_ldap` auth backend module is configured. However, the `rabbitmq_auth_backend_ldap` plugin must be enabled in order to use this auth backend. Until then it will be skipped during authentication/authorization

* Warning message when some auth backends were filtered out:

    [warning] <0.213.0> Some configured backends were dropped because their corresponding plugins are disabled. Please look at the info messages above to learn which plugin(s) should be enabled. Here is the list of auth backends kept after filering:
    [warning] <0.213.0> [rabbit_auth_backend_internal]

* Error message when no auth backends are valid:

    [error] <0.213.0> None of the configured auth backends are usable because their corresponding plugins were not enabled. Please look at the info messages above to learn which plugin(s) should be enabled.

V2: In fact, `rabbit_plugins:is_enabled/1` indicates if a plugin is
    running, not if it is enabled... The new check runs as a boot step
    and thus is executed before plugins are started. Therefore we can't
    use this API. Instead, we use `rabbit_plugins:enabled_plugins/0'
    which lists explicitly enabled plugins. The drawback is that in the
    auth backend is enabled implicitly because it is a dependency of
    another explicitly enabled plugin, the check will still consider it
    is disabled and thus abort the boot.

Fixes #13783.