feat: handling x-www-form-urlencoded

[maximeroucher]

closes #632

Description
Hello,

I discovered this project, and it looked awesome, so I used it in order to replace my custom API call handling functions. It works perfectly for application/json.
However, my API implements OIDC, and thus uses application/x-www-form-encoded instead of application/json.
When running, the generated code for those API routes was throwing error.
After some research, I found that this issue has already been reported, so I am proposing a fix for it.

How does it work?
Until now, x-www-form-encoded were handled the same way as multipart (in swagger_models/requests/swagger_request.dart).
I created a new parameter called isUrlencoded to dissociate these two cases, and used to it handle the right way specified here : https://hadrien-lejard.gitbook.io/chopper/requests#sending-application-x-www-form-urlencoded-data

Checklist
[x] I have performed a self-review of my code
[x] My code follows the code style of this project
[x] I have commented my code, particularly in hard-to-understand areas
If you think this change may be interesting to add to swagger-dart-code-generator, I will be happy to improve the pull request based on received feedback and suggestions.

Best regards

[Vovanella95]

Hi @maximeroucher , thanks for contribution

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (f874a5d) 55.73% compared to head (2d2fd0c) 54.92%.
Report is 5 commits behind head on master.

Files	Patch %	Lines
...rc/code_generators/swagger_requests_generator.dart	94.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #687      +/-   ##
==========================================
- Coverage   55.73%   54.92%   -0.81%     
==========================================
  Files          10       10              
  Lines        1229     1238       +9     
==========================================
- Hits          685      680       -5     
- Misses        544      558      +14     

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[Vovanella95]

@maximeroucher please resolve conflicts

[maximeroucher]

Here you go !