Make getting obo context prefix map easier

[cthoyt]

This PR exposes bioregistry.get_obo_context_prefix_map() for use in INCATools/ontology-access-kit#224.

[codecov-commenter]

Codecov Report

Merging #501 (4f23b53) into main (3e6eabc) will decrease coverage by 0.15%.
The diff coverage is 61.76%.

@@            Coverage Diff             @@
##             main     #501      +/-   ##
==========================================
- Coverage   37.59%   37.44%   -0.16%     
==========================================
  Files         109      109              
  Lines        6057     6060       +3     
  Branches     1234     1234              
==========================================
- Hits         2277     2269       -8     
- Misses       3637     3651      +14     
+ Partials      143      140       -3     

Impacted Files	Coverage Δ
src/bioregistry/__init__.py	100.00% <ø> (ø)
src/bioregistry/export/prefix_maps.py	0.00% <0.00%> (-43.55%)	⬇️
src/bioregistry/schema/struct.py	82.07% <50.00%> (-0.10%)	⬇️
src/bioregistry/collection_api.py	90.00% <66.66%> (-10.00%)	⬇️
src/bioregistry/resource_manager.py	80.78% <88.23%> (+1.55%)	⬆️
src/bioregistry/app/api.py	52.38% <100.00%> (-0.57%)	⬇️
src/bioregistry/resolve.py	63.40% <100.00%> (-0.12%)	⬇️
src/bioregistry/uri_format.py	85.71% <0.00%> (-7.15%)	⬇️
... and 1 more

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