Add a $sample accumulator operator

So that it's easier to sample a number of items from each group, instead of writing lengthy DSL like this: https://www.mongodb.com/community/forums/t/sample-x-number-of-documents-in-each-group-with-or-after-a-group-stage/170787 Ideally it can be expressed like this: ` { $group: { _id: '$year', samples: { $sample: 100 }, } } ` Hence making sampling 100 items from each group a snap to achieve!