Add interfaces for making trust nodes in TheoryInferenceManager. (#5016)

Add interfaces for making trust nodes in TheoryInferenceManager. (#5016)

This gives theories a finer grained control over explained lemmas and conflicts.

A theory may now use an inference manager to construct "explained" lemmas/conflicts e.g. via mkLemmaExp, subsequently do any theory-specific debugging or modification to that lemma before sending it via trustedLemma.

This is required for the new strings inference manager on proof-new.

This also adds a missing variant of conflicts for the proof equality engine. It also does a minor simplification of a previous variant for constructing conflicts from proof equality engine based on a proof step buffer.