Add readonly[const] model to each collection's concepts

https://github.com/linksplatform/Interfaces/blob/6c1408ecfb63e1e29059e0807630ef44b7d1b99c/cpp/Platform.Interfaces/IDictionary.h#L8
This is what a small hack looks like
You need to define readonly models that use only const methods

Start with cpp/Platform.Interfaces/IDictionary.h at the linked definition, then inspect the other collection concepts that need corresponding readonly models. Define readonly models whose methods are const, and verify that every collection concept has the requested readonly form.

Written by the indexing model from the issue text.