Allow to setup schema format of the field

[andreykurilin]

Raw object has schema_format class-level property that is used while constucting jsonschema. Making it configurable while describing an obj allows to avoid constructing empty python classes that overrides just one property.

[codecov[bot]]

Codecov Report

Merging #216 (2890edf) into master (84ae836) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #216   +/-   ##
=======================================
  Coverage   96.85%   96.85%           
=======================================
  Files          19       19           
  Lines        2705     2707    +2     
=======================================
+ Hits         2620     2622    +2     
  Misses         85       85           

Impacted Files	Coverage Δ
flask_restx/fields.py	96.60% <100.00%> (+0.01%)	:arrow_up:

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