Allow plugins to access `pulp_plugin_configs` and `get_plugin_config` from `pulpcore.app.apps`

[evgeni]

Is your feature request related to a problem? Please describe.

The pulp_smart_proxy plugin exposes a Pulp installation to Foreman/Katello as a standalone "smart proxy" (the Foreman Proxy API). To do so, it needs to be able to inform Foreman/Katello about details of the Pulp installation: the version, the available content plugins, etc.

To do so, we pull the list of enabled plugins via pulpcore.app.apps.pulp_plugin_configs() and versions via pulpcore.app.apps.get_plugin_config(NAME).version but this is not allowed per the API contract between Pulp and it's plugins.

Describe the solution you'd like
Expose pulp_plugin_configs and get_plugin_config via pulpcore.plugin.apps like it was done for adjust_roles in 2e4a200

Describe alternatives you've considered

• call the status REST api from within the plugin - lolnope
• ignore your new lint check - nope

Additional context
<3 for all the nice automation you have for plugin autors