Offline third party feature contribution

Before actually diving into building a Hippo Marketplace, we want to allow partners to contribute to Essentials in some 'offline' way. We want them to be able to give us a plugin or a power pack, and we ship it with Essentials or we offer it purely as a description in the library. The customer will then have to contact the partner for the actual code.

Also, we need to know how we'll extend this in the future so we can share the Marketplace roadmap with our partners.

Needs to work with existing 7.9 stack.