Feature request: import / export content rules

Plone content rules are stored in an object `IRuleStorage` which is not visible in Zope space and is also not visible in content space (it can only be obtained using `getUtility()`). Implementation-wise, `IRuleStorage` is implemented as a simple `OOBTree()`, that can be acquired (`plone.app.contentrules.browser.assignments.acquired_rules()` has the scoop).

It would be great if the import/export framework had a step to deal with the rule storage. This would require at least a custom pair of serializer/deserializer.

Start by reading the import/export framework and the IRuleStorage implementation, using plone.app.contentrules.browser.assignments.acquired_rules() as the referenced entry point. Determine how the OOBTree-backed rule storage is acquired, then define the serializer/deserializer pair and framework step needed to import and export it. Done means rule storage is handled by the import/export framework.

Written by the indexing model from the issue text.