OpenAPI app and high level integration

[p1c2u]

Spec object creation no longer validates specification (moved to OpenAPI object).

Fixes #547 #714

[codecov]

Codecov Report

Attention: 30 lines in your changes are missing coverage. Please review.

Comparison is base (46639b5) 91.20% compared to head (b315e76) 91.42%.
Report is 8 commits behind head on master.

Files	Patch %	Lines
openapi_core/validation/processors.py	0.00%	8 Missing ⚠️
openapi_core/validation/integrations.py	78.12%	7 Missing ⚠️
openapi_core/contrib/django/middlewares.py	54.54%	4 Missing and 1 partial ⚠️
openapi_core/app.py	98.27%	3 Missing and 1 partial ⚠️
openapi_core/contrib/falcon/integrations.py	90.00%	1 Missing and 1 partial ⚠️
openapi_core/contrib/starlette/integrations.py	93.33%	1 Missing and 1 partial ⚠️
openapi_core/unmarshalling/integrations.py	95.12%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #716      +/-   ##
==========================================
+ Coverage   91.20%   91.42%   +0.22%     
==========================================
  Files         129      139      +10     
  Lines        4129     4375     +246     
  Branches      507      491      -16     
==========================================
+ Hits         3766     4000     +234     
- Misses        296      313      +17     
+ Partials       67       62       -5     

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