Implements Swagger apiKey Preauthorization

[yweber]

Solution proposal for #196

The flask_restx.Api class provides an apikey_preauthorization decorator allowing to register a function returning wether or not preauthorization is wanted.

In the case where preauthorization is wanted, the registered function should returns a tuple with Authorization_name and the APIKey.

[codecov[bot]]

Codecov Report

Merging #198 into master will decrease coverage by 0.06%. The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master     #198      +/-   ##
==========================================
- Coverage   96.92%   96.85%   -0.07%     
==========================================
  Files          19       19              
  Lines        2697     2701       +4     
==========================================
+ Hits         2614     2616       +2     
- Misses         83       85       +2     

Impacted Files	Coverage Δ
flask_restx/api.py	96.69% <50.00%> (-0.45%)	:arrow_down:
flask_restx/apidoc.py	100.00% <100.00%> (ø)

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