replace set with addFields in aggregations py

[AaronDou]

What changes are proposed in this pull request?

This PR replaces the aggregation stage $set operator with the $addFields operator in aggregations.py. These two operators are alias to each other in MongoDB. The reason for the change is $addFields is supported by AWS DocumentDB while $set is not.

Note the $set operator is overloaded in MongoDB, and the one under change is the aggregation stage $set operator as opposed to the update operator $set.

After this PR is approved, follow up PRs will be submitted to replace all the $set aggregation operators to $addFields across the codebase to keep things consistent.

How is this patch tested? If it is not, please explain why.

pytest tests/unittests all green.
No new tests are added as this is effectively a cosmetic change.

Release Notes

Is this a user-facing change that should be mentioned in the release notes?

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• Yes. Give a description of this change to be included in the release
  notes for FiftyOne users.

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What areas of FiftyOne does this PR affect?

• App: FiftyOne application changes
• Build: Build and test infrastructure changes
• Core: Core fiftyone Python library changes
• Documentation: FiftyOne documentation changes
• Other

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.01 ⚠️

Comparison is base (7f5d87d) 48.61% compared to head (9e0f357) 48.61%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3111      +/-   ##
===========================================
- Coverage    48.61%   48.61%   -0.01%     
===========================================
  Files          227      227              
  Lines        33355    33359       +4     
  Branches       319      319              
===========================================
  Hits         16217    16217              
- Misses       17138    17142       +4     

Flag	Coverage Δ
app	48.61% <0.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ages/components/src/components/Results/Results.tsx	29.80% <0.00%> (-1.20%)	⬇️

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[brimoor]

LGTM ✔️ @AaronDou note that I grabbed all the other instances of $set and $unset in #3174