REF: Rename _MakerCRS to CustomConstructorCRS and make public

[snowman2]

With the change in #902, the base class CustomConstructorCRS is useful for other libraries such as cartopy which have a different __init__:

• REF: Use pyproj 3+ for PROJ 8+ migration SciTools/cartopy#1808 (comment)
• REF: Use pyproj 3+ for PROJ 8+ migration SciTools/cartopy#1808 (comment)

[codecov]

Codecov Report

Merging #912 (1a94461) into master (c4cdc33) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #912      +/-   ##
==========================================
+ Coverage   96.18%   96.23%   +0.05%     
==========================================
  Files          21       21              
  Lines        1755     1755              
==========================================
+ Hits         1688     1689       +1     
+ Misses         67       66       -1     

Impacted Files	Coverage Δ
pyproj/crs/__init__.py	100.00% <ø> (ø)
pyproj/crs/crs.py	97.98% <100.00%> (+0.20%)	⬆️

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