To aid in support cases and allow explain to convey information about the cached plan (since usually it is ignored), we should add an "isCached" field to all plans listed in explain. This will be a boolean to indicate whether the plan matches the cached plan.

For support cases where a bad plan is cached and the user runs explain and sees a different plan reported, the issue will be more obvious. We can also add a top level field "doesWinningPlanMatchCached" to surface this even higher.