Enable applying an assignment plugin multiple times in a bundle

Problem/Motivation

Currently we support applying a given assignment plugin a maximum of one time per features bundle. While this covers many use cases, there are more complex requirements that can't be met this way.

For example, the base assignment plugin, assigned a low weight, is useful for creating one feature per content type, with field storages specific to each content type being assigned to the content type based features. But there is also a use case for non-specific field storages to be added to their own features. To support both use cases, we need to add the base assignment plugin twice, once with a low weight and once with a higher one.

Proposed resolution

Enable applying an assignment plugin multiple times in a bundle.

Remaining tasks

User interface changes

API changes

Data model changes