[mlir][NFC] Expose the `CFGToSCFInterface` for the `ControlFlowToSCF` pass

This is useful for any downstream users who may just want to use slightly different ops than the pass or need to take into account other ops in the input while still dealing with ControlFlow ops for the most part.
This also helps writing test implementations for `transformCFGToSCF`.

Since the implementation is now public, comments noting the post conditions of the methods have been added to ensure downstream users can stay compatible with the existing implementation.

Differential Revision: https://reviews.llvm.org/D157971