Allow plugins to register generic assets capacities

[cedric-anne]

Checklist before requesting a review

• I have read the CONTRIBUTING document.
• I have performed a self-review of my code.
• I have added tests that prove my fix is effective or that my feature works.
• This change requires a documentation update.

Description

This provides a method that can be used by plugins to register asset capacities from their setup function.

AssetDefinitionManager::getInstance()->registerCapacity(new MyPluginCapacity());