add interface for custom cell body persistence provider

[jingwood]

Fixes

• Fixes the issue DropdownListCell body type is not serializing/deserializing in any format when saving/loading workbook #388

The new API

Added the interface to provide the custom cell body data handler during saving and loading into RGF format.

RGFPersistenceProvider.CustomBodyTypeProviders.Add(typeof(MyCellBody), "my-cell-body", new MyCellBodyPersistenceProvider());