Implement dataless extraction, collapsing and aggregation

[ESadek-MO]

🚀 Pull Request

Description

Closes #6725.

• Extract
• Collapse
• Aggregate_by
• Tests
• Docs
• Whatsnew

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[codecov]

Codecov Report

❌ Patch coverage is 92.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.29%. Comparing base (ecab3df) to head (1174bbb).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/iris/cube.py	92.42%	1 Missing and 4 partials ⚠️
lib/iris/util.py	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6724   +/-   ##
=======================================
  Coverage   90.29%   90.29%           
=======================================
  Files          91       91           
  Lines       24476    24497   +21     
  Branches     4571     4578    +7     
=======================================
+ Hits        22101    22120   +19     
+ Misses       1607     1606    -1     
- Partials      768      771    +3     

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