Move casters to separate subpackage

[p1c2u]

Next step in package cleanup/refactoring

[codecov]

Codecov Report

Merging #184 into master will decrease coverage by 0.14%.
The diff coverage is 91.22%.

@@            Coverage Diff             @@
##           master     #184      +/-   ##
==========================================
- Coverage   96.32%   96.17%   -0.15%     
==========================================
  Files          84       88       +4     
  Lines        1794     1829      +35     
==========================================
+ Hits         1728     1759      +31     
- Misses         66       70       +4

Impacted Files	Coverage Δ
openapi_core/schema/schemas/exceptions.py	94.44% <ø> (+2.77%)	⬆️
openapi_core/schema/media_types/models.py	90.69% <100%> (+0.22%)	⬆️
openapi_core/casting/schemas/factories.py	100% <100%> (ø)
openapi_core/schema/schemas/models.py	97.77% <100%> (-0.21%)	⬇️
openapi_core/schema/parameters/models.py	97.22% <100%> (+0.03%)	⬆️
openapi_core/casting/schemas/util.py	50% <50%> (ø)
openapi_core/casting/schemas/exceptions.py	87.5% <87.5%> (ø)
openapi_core/casting/schemas/casters.py	95.23% <95.23%> (ø)
... and 2 more

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