oci: casext: mediatype: switch to generics for parser functions

[cyphar]

We cannot use generics for the parser function map, but we can use it when instantiating "simple" JSON parser functions. There are a few other downsides to Go's generics, so this should just be a minor (possibly even theoretical) performance improvement over constructing the parsed types using reflection.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 81.81818% with 6 lines in your changes missing coverage. Please review.

Project coverage is 73.68%. Comparing base (3405ba8) to head (71d012d).
Report is 21 commits behind head on main.

Files with missing lines	Patch %	Lines
oci/casext/mediatype/parse.go	81.81%	4 Missing and 2 partials ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##             main     #476       +/-   ##
===========================================
+ Coverage   46.37%   73.68%   +27.31%     
===========================================
  Files          60       60               
  Lines        4895     4903        +8     
===========================================
+ Hits         2270     3613     +1343     
+ Misses       2357      941     -1416     
- Partials      268      349       +81     

Files with missing lines	Coverage Δ
oci/casext/mediatype/parse.go	76.69% <81.81%> (+5.12%)	⬆️

... and 32 files with indirect coverage changes