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]

Codecov Report

Merging #476 (11a8818) into main (899f30e) will increase coverage by 0.04%. The diff coverage is 81.81%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #476      +/-   ##
==========================================
+ Coverage   73.34%   73.38%   +0.04%     
==========================================
  Files          60       60              
  Lines        4881     4889       +8     
==========================================
+ Hits         3580     3588       +8     
- Misses        940      941       +1     
+ Partials      361      360       -1     

Impacted Files	Coverage Δ
oci/casext/mediatype/parse.go	76.69% <81.81%> (+1.96%)	:arrow_up: