Add new function(s) that can extract the skeleton of an explain output and enable comparison of two explain outputs via golden testing. The resulting output should contain relevant data for comparison, i.e. explainVersion, winningPlan stages, rejectedPlans, etc. but no unique information to each run i.e. serverInfo, optimization time, etc. Currently we only have getPlanSkeleton which is similar in concept.

This idea arose when testing explain output for classic multiplanner + SBE, as there is no simple way to test that the joint explain output contains relevant sections of both classic and sbe's respective explain outputs.