(proof-new) Add interface for trusted substitution and update ppAssert (#5193)

(proof-new) Add interface for trusted substitution and update ppAssert (#5193)

The current work on proof-new involves proofs for preprocessing. The biggest issue currently is that our preprocessing passes do not track proofs for substitutions.

This adds a "trusted substitution" class with is a layer on substitution map. The proof aspect of this class is not yet implemented, this PR just adds its interface.

This also updates Theory::ppAssert to take a TrustSubstitutionMap instead of a SubstitutionMap, since eventually we will require proofs to be provided for substitutions that are added to this map.