Add missing methods to Solver API for models (#7052)

Add missing methods to Solver API for models (#7052)

This adds the last two remaining API methods required for implementing the text output of get-model on the API side.

A followup PR will implement GetModelCommand on the API side and eliminate the (last remaining) call to getSmtEngine() from the command layer.

This PR does some minor reorganization of the model cores code to account for the new interface. It also removes a deprecated interface from TheoryModel.