use $addFields and $project in aggregation pipelines

[brimoor]

Replaces all usages of $set and $unset in aggregation pipelines with $addFields and $project: {XXX: False}, respectively.

This is being done as part of an effort to broaden the compatibility of FiftyOne with older MongoDB versions and other technologies like DocumentDB.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (7c00686) 49.22% compared to head (bf131b1) 49.22%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3174   +/-   ##
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  Coverage    49.22%   49.22%           
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  Files          230      230           
  Lines        34446    34446           
  Branches       325      325           
========================================
  Hits         16957    16957           
  Misses       17489    17489           

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[swheaton]

Looks good to me!

For future readers, here's the documentDB compatibility page

Side note for future, another stage we use that is unsupported by Document DB is $merge. But that one will be much harder to replace than $set and $unset I imagine.

[swheaton]

There are also many ViewExpression functions that are incompatible such as trim and ceil. Probably not as big of a deal for it to just not work in doc db.

Also $count as an accumulator and $$CURRENT. I'm sure there's others.