PHPLIB-1654: Support new $concatArrays and $setUnion aggregation accumulators

[paulinevos]

Closes PHPLIB-1654

Does what it says on the tin.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 87.54%. Comparing base (ae31f00) to head (5f6ab18).

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...rc/Builder/Accumulator/ConcatArraysAccumulator.php	66.66%	2 Missing ⚠️
src/Builder/Accumulator/SetUnionAccumulator.php	66.66%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               v2.x    #1732      +/-   ##
============================================
- Coverage     87.57%   87.54%   -0.04%     
- Complexity     3174     3182       +8     
============================================
  Files           421      423       +2     
  Lines          6320     6336      +16     
============================================
+ Hits           5535     5547      +12     
- Misses          785      789       +4     

Flag	Coverage Δ
6.0-replica_set	85.65% <75.00%> (-0.03%)	⬇️
6.0-server	81.66% <75.00%> (-0.02%)	⬇️
6.0-sharded_cluster	85.24% <75.00%> (-0.03%)	⬇️
8.0-replica_set	87.46% <75.00%> (-0.04%)	⬇️
8.0-server	82.54% <75.00%> (-0.02%)	⬇️
8.0-sharded_cluster	87.13% <75.00%> (-0.04%)	⬇️

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[alcaeus]

LGTM with a minor comment.

Looking at the coverage, it's missing tests for exceptions when providing invalid arguments. We can leave those out for the time being and I'd rather add some kind of semi-automated testing like we have for operation classes (see this code for an example).

[alcaeus]

Ha, our code from discussing different ways to write agg pipelines found its way in here. I'll let you decide whether you want to keep it this way or inline it.

[paulinevos]

I liked this :)