Bridge support

[pp-mo]

NOT FOR MERGE

A preliminary attempt at combining #5212 and #5214, rebased onto recent main branch, for testing "bridge" code
( as in #4994 ).

N.B. 2f4751d indicates that we may have a bit more work to successfully integrate these changes with those from #5095 .

[codecov]

Codecov Report

Patch coverage: 90.42% and project coverage change: +0.04 🎉

Comparison is base (7dfc29a) 89.27% compared to head (2f4751d) 89.31%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5216      +/-   ##
==========================================
+ Coverage   89.27%   89.31%   +0.04%     
==========================================
  Files          88       88              
  Lines       22258    22284      +26     
  Branches     4867     4877      +10     
==========================================
+ Hits        19871    19904      +33     
+ Misses       1641     1637       -4     
+ Partials      746      743       -3     

Impacted Files	Coverage Δ
lib/iris/fileformats/netcdf/saver.py	89.97% <81.81%> (+0.05%)	⬆️
lib/iris/__init__.py	88.88% <83.33%> (+0.36%)	⬆️
lib/iris/fileformats/__init__.py	84.37% <100.00%> (-0.48%)	⬇️
lib/iris/fileformats/cf.py	85.65% <100.00%> (+0.11%)	⬆️
lib/iris/fileformats/netcdf/loader.py	80.29% <100.00%> (+0.29%)	⬆️
lib/iris/io/__init__.py	82.97% <100.00%> (+2.09%)	⬆️
lib/iris/io/format_picker.py	83.72% <100.00%> (+2.26%)	⬆️

... and 1 file with indirect coverage changes

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[pp-mo]

No longer needed -- the required support is now in 'main'