Better handling of content types

[cthoyt]

Closes biopragmatics/bioregistry#775.

This PR adds handling of headers to both the Flask and FastAPI implementations of the apps.

• Add Flask implementation
• Add FastAPI implementation
• Add Flask tests
• Add FastAPI tests
• Should the output parameter be supported?

CC @vemonet. Ideally, I'd like to use https://github.com/vemonet/rdflib-endpoint and not re-implement this code, but we'll have to work through a few issues first (improving code modularity, documentation, and figuring out Flask suppot) before I can give that a try

[codecov-commenter]

Codecov Report

Merging #46 (f3ba196) into main (89ef83a) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #46   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines          443       453   +10     
  Branches        94        95    +1     
=========================================
+ Hits           443       453   +10     

Impacted Files	Coverage Δ
src/curies/mapping_service/__init__.py	100.00% <100.00%> (ø)

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