Add nested related filters to docs schema

[Ken4scholars]

Currently, nested filters are not shown in swagger docs because they are not added to the OpenAPI schema. With this PR, the filters are expanded and added when generating the docs

[codecov-io]

Codecov Report

Merging #320 into 0.11.x will decrease coverage by 5.26%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           0.11.x     #320      +/-   ##
==========================================
- Coverage   98.33%   93.06%   -5.27%     
==========================================
  Files           4        4              
  Lines         180      202      +22     
  Branches       41       46       +5     
==========================================
+ Hits          177      188      +11     
- Misses          1       11      +10     
- Partials        2        3       +1

Impacted Files	Coverage Δ
rest_framework_filters/backends.py	67.74% <23.07%> (-32.26%)	⬇️
rest_framework_filters/filterset.py	98.41% <88.88%> (-0.74%)	⬇️

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[Ken4scholars]

Please review and let me know if I missed anything. This is my first time opening a PR here

[rpkilby]

Hi @Ken4scholars. Apologies that I'm going to be a bit terse, but I can go into more detail if you have questions. I left a few comments below, but additionally:

• Schema support should be tested.
• Highly nested and recursive relationships should be considered (i.e., how deep do you go?). By default, the backend renders forms with a depth of 1. Look into disable_subset.

[Ken4scholars]

Hello @rpkilby. Thank you for the review and pointing out things that should be considered.

• I will try to add tests for the schema generation
• Will also look at the recursion depth. For now, it retrieves every filter down to the tree leaves. In cases like AllFilter that is sure going to bloat the docs so I will check how the forms are implemented and use a similar strategy.

[rpkilby]

Hi @Ken4scholars. Given that the PR is targeting 0.11.x, I'm going to go ahead and close it. Happy to reconsider a PR targeting the master branch and django-filter 2.

[Ken4scholars]

Hello @rpkilby sorry, but any reason why there shouldn't be new PRs for 0.11x? 1.00 is still in dev mode and has not been released yet. Is it a project policy? Just to be clear, I considered this a fix and not a new feature and that's why I decided to push it to the latest branch in use instead of waiting for the next big release.