Preparation for list-struct backend for NestedExtensionArray

[hombit]

This PR adds a couple of helper functions to prepare a transition from struct-list chunked array to list-structure chunked array as the storage of NestedExtensionArray data.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.27%. Comparing base (626718e) to head (5738621).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #242      +/-   ##
==========================================
+ Coverage   98.17%   98.27%   +0.09%     
==========================================
  Files          14       14              
  Lines        1316     1331      +15     
==========================================
+ Hits         1292     1308      +16     
+ Misses         24       23       -1     

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[github-actions]

Before [626718e]	After [344c4c3]	Ratio	Benchmark (Parameter)
35.7±5ms	42.7±8ms	~1.20	benchmarks.ReassignHalfOfNestedSeries.time_run
9.88±0.08ms	10.1±0.1ms	1.03	benchmarks.NestedFrameAddNested.time_run
25.0±1ms	25.2±0.6ms	1.01	benchmarks.AssignSingleDfToNestedSeries.time_run
10.8±0.07ms	10.9±0.05ms	1.01	benchmarks.NestedFrameQuery.time_run
1.30±0.04ms	1.31±0.01ms	1.01	benchmarks.NestedFrameReduce.time_run
274M	274M	1.00	benchmarks.AssignSingleDfToNestedSeries.peakmem_run
97M	97.2M	1.00	benchmarks.NestedFrameAddNested.peakmem_run
101M	101M	1.00	benchmarks.NestedFrameQuery.peakmem_run
101M	101M	1.00	benchmarks.NestedFrameReduce.peakmem_run
293M	293M	1.00	benchmarks.ReassignHalfOfNestedSeries.peakmem_run

Click here to view all benchmarks.

[hombit]

Commit list is way too long, I just wanted to take few changes from the previous implementation attempt.

[dougbrn]

Looks good, just one docs request