Support by reduction within summary reduction

[ianthomas23]

Fixes #1252, allowing use of by reduction within a summary reduction such as

ds.summary(other=ds.by("cat"))`

[codecov]

Codecov Report

Merging #1254 (e4bcb0d) into main (7bff9b4) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1254      +/-   ##
==========================================
+ Coverage   83.42%   83.44%   +0.02%     
==========================================
  Files          35       35              
  Lines        8869     8882      +13     
==========================================
+ Hits         7399     7412      +13     
  Misses       1470     1470              

Impacted Files	Coverage Δ
datashader/compiler.py	88.38% <100.00%> (+0.05%)	⬆️
datashader/reductions.py	78.06% <100.00%> (+0.18%)	⬆️

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