Update ObservableProxy::pipe to support any number of operators

[cwharris]

Description

Adds support for unlimited pipe arguments.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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[cwharris]

/ok to test

[cwharris]

/ok to test

[codecov]

Codecov Report

Merging #387 (1702de9) into branch-23.11 (40d20a3) will increase coverage by 0.02%.
The diff coverage is 83.33%.

❗ Current head 1702de9 differs from pull request most recent head bce621b. Consider uploading reports for the commit bce621b to get more accurate results

Additional details and impacted files

@@               Coverage Diff                @@
##           branch-23.11     #387      +/-   ##
================================================
+ Coverage         73.35%   73.38%   +0.02%     
================================================
  Files               385      382       -3     
  Lines             13663    13354     -309     
  Branches           1025     1012      -13     
================================================
- Hits              10023     9800     -223     
+ Misses             3640     3554      -86     

Flag	Coverage Δ
cpp	69.35% <0.00%> (-0.06%)	⬇️
py	42.22% <83.33%> (+0.29%)	⬆️

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

Files Changed	Coverage Δ
python/mrc/_pymrc/src/subscriber.cpp	24.52% <83.33%> (+7.86%)	⬆️

... and 33 files with indirect coverage changes

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[cwharris]

/merge