add django rest framework mongoengine support

[ngocngoan]

• Add class MongoEngineAutoSchema, which inherits from class AutoSchema and is specified at the entry DEFAULT_SCHEMA_CLASS in the configuration

REST_FRAMEWORK = {
    ...
    "DEFAULT_SCHEMA_CLASS": "drf_spectacular.openapi.MongoEngineAutoSchema",
    ...
}

• Add new function get_mongoengine_extended_doc_excludes that compatible with mongoengine, specify this function in the config

SPECTACULAR_SETTINGS = {
    "GET_LIB_DOC_EXCLUDES": 'drf_spectacular.plumbing.get_mongoengine_extended_doc_excludes',
}

[tfranzel]

thanks for the PR. that is awesome! couple of things before i can really review it.

• this breaks everyone not using mongoengine, which is a no-go. code must always work for regular DRF users.
• all this functionality is contrib and therefore goes into contrib/django_rest_framework_mongoengine.py
• no added deps into main.txt, these are optionals and please only a minimal set. it would install mongoengine for every spectacular user.
• unit tests missing. please refer to other contrib packages for how the tests work here. it does not have to be fancy but something that at least tests basic functionality.

[ngocngoan]

I will consider your terms. When the code is completed, I will notify you.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 29 lines in your changes missing coverage. Please review.

Project coverage is 96.84%. Comparing base (1a49ddf) to head (bb3c6f2).
Report is 842 commits behind head on master.

Files	Patch %	Lines
...cular/contrib/django_rest_framework_mongoengine.py	0.00%	29 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #181      +/-   ##
==========================================
- Coverage   97.60%   96.84%   -0.77%     
==========================================
  Files          49       50       +1     
  Lines        3672     3703      +31     
==========================================
+ Hits         3584     3586       +2     
- Misses         88      117      +29     

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[tfranzel]

@ngocngoan thanks! that looks so much better!

in order to merge this, we need to test the functionality. here is the test script i created when you first opened the issue, in order to check if this is even remotely working:
https://gist.github.com/tfranzel/c11382ea8e3801b701de0f4ef73fe0d5

feel free to adapt. basic functionality should be included.

[tfranzel]

pymongo is not required as it is a dependency of mongoengine

mongomock will likely also come in handy for testing