Added union method to HLL

[LucaCappelletti94]

• While several other methods relative to the joint estimation were available, union estimation was not.
• Added union method and relative test suite addition.

[LucaCappelletti94]

@ctb this one should be more straightforward than PR #3292 as it does not introduce any new dependency.

[ctb]

@luizirber is this something you can review?

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 86.74%. Comparing base (bc22970) to head (ed67c55).
Report is 69 commits behind head on latest.

Files with missing lines	Patch %	Lines
src/core/src/sketch/hyperloglog/mod.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           latest    #3293      +/-   ##
==========================================
- Coverage   86.75%   86.74%   -0.02%     
==========================================
  Files         137      137              
  Lines       15920    15923       +3     
  Branches     2728     2728              
==========================================
+ Hits        13812    13813       +1     
- Misses       1799     1801       +2     
  Partials      309      309              

Flag	Coverage Δ
hypothesis-py	25.40% <ø> (ø)
python	92.37% <ø> (ø)
rust	62.64% <66.66%> (-0.03%)	⬇️

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[luizirber]

Thanks @LucaCappelletti94 !