Added Service.oas_file field to save copy of OAS instead of fetching by URL

[Bartvaderkin]

Refs: open-formulieren/open-forms#920

[codecov-commenter]

Codecov Report

Merging #25 (c468a33) into master (9b0759d) will increase coverage by 2.12%.
The diff coverage is 78.94%.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
+ Coverage   35.50%   37.62%   +2.12%     
==========================================
  Files          29       29              
  Lines        1090     1103      +13     
  Branches      154      156       +2     
==========================================
+ Hits          387      415      +28     
+ Misses        680      658      -22     
- Partials       23       30       +7     

Impacted Files	Coverage Δ
zgw_consumers/client.py	71.79% <66.66%> (+13.73%)	⬆️
zgw_consumers/models.py	61.06% <100.00%> (+13.83%)	⬆️

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[Bartvaderkin]

I thought about renaming the existing .oas URLField to .oas_url but decided against it for compatibility reasons.

[Bartvaderkin]

I removed the unnecessary null attribute.