Refactor collectModelInfo in TheoryArith (#5027)

Refactor collectModelInfo in TheoryArith (#5027)

This is work towards updating the arithmetic solver to the new standard, and in particular isolating TheoryArithPrivate as the "linear solver", and TheoryArith as the overall approach for arithmetic.

This transfers ownership of the non-linear extension from TheoryArithPrivate to TheoryArith. The former still has a pointer to the non-linear extension, which will be removed with further refactoring.

This PR additionally moves the code that handles the interplay of the non-linear extension in TheoryArithPrivate::collectModelInfo to TheoryArith, and simplifies the model interface for TheoryArithPrivate.