As a user, I can configure Pulp to include routes from third-party django apps

Author: @bmbouter (bmbouter)

Redmine Issue: 8898, https://pulp.plan.io/issues/8898

Problem

Some users want to use include third party software, e.g. django-sentry or django-social-auth.

Solution

Introduce a setting named ADDITIONAL_URLS which defaults to ADDITIONAL_URLS = None.

Users could solve this django-sentry inclusion by specifying:

<Details here>

Alternatives Considered

Pulp could attempt to include urls of various common third-party applications as users request them, like this commit would have. This has several drawbacks:

1. a release must occur for any new request
2. The user may want it rooted somewhere else
3. There is a low-grade security concern that gratuitously importing third-party Python code without users being able to turn it off may not be safe, for example with package name mimicking

Another alternative was to have settings to enable specific third-party url inclusion. This has these drawbacks though:

1. a release must occur for any new request
2. The user may want it rooted somewhere else

Start by tracing pulpcore's existing Django URL configuration and settings handling. Determine how ADDITIONAL_URLS should default to None and how users specify third-party Django routes and their mount points. Done means the setting works for the described inclusion use case and is covered by appropriate tests or documentation.