emit an $addFields stage before $sort with safe aliases if necessary

[hallettj]

This fixes sorting on field names that contain dollar signs or dots. It also provides a framework that we can use to sort on aggregation results.

Unlike $match (which has the $expr operator), the $sort stage does not have any way to express arbitrary expressions. That is necessary to use the $getField operator which is what we use to reference field names that need to be escaped. So what this PR does is to inject an $addFields stage before the $sort stage if necessary that creates aliases that are safe to use as keys in $sort.