IObjectSerializer should not be used outside Shell

Description

IObjectSerializer was intended for the Shell layer. It could have specific type converters registered to support the UI. It should not be used in other places, like serializing data for specialized properties. The CMS Core team has written IObjectSerializer out of their code and are planning to move it to the Shell assembly in the next major release.

Commerce uses IObjectSerializer in a number of specialized properties derived from PropertyCollectionBase<T> and also for List<T> handling in MetaDataPropertyMapper. These should be changed to remove the dependency to IObjectSerializer as the CMS Core team has done.