Add Propshaft server rendering support

[elektronaut]

Summary

Server rendering currently raises an undefined method '[]' for an instance of Propshaft::Assembly error when using the Propshaft assets pipeline, since it assumes Sprockets is present.

This PR adds a new PropshaftContainer adapter. According to the Propshaft documentation, this API works the same for both precompiled and dynamic assets.

Pull Request checklist

• [x] Add/update test to cover these changes
• [x] Update documentation
• [x] Update CHANGELOG file