[proof-new] Registering proof checkers uniformly from the SMT solver (#6275)

[proof-new] Registering proof checkers uniformly from the SMT solver (#6275)

Each theory has its own proof checker, responsible for checking the rules pertaining to that theory. The main proof checker uses these specialized checkers. Previously the main proof checker (of the proof node manager used across the SMT solver) was connected to these theory proof checkers during initialization of the theory. This commit adds an interface to the theories for retrieving its proof checker (analogous to how one retrieves the rewriter of a theory) which is used by a new method in the theory engine to register a theory proof checker to a given proof checker according to a theory id.

This is in preparation for the new unsat cores based on proofs.