Django filter typing improvements

[tfranzel]

continuation of carltongibson/django-filter#1479

@jeking3 would you mind reviewing this? specifically the impact on your schema

• now allowing extend_schema_field decoration of filter methods (in addition to wrapping the field itself)
• also raise priority (but lower than decoration) of type hints on filter methods since it is a deliberate attempt to provide more information. before, most hints to value were simply not considered.

fyi users of this extension, feedback welcome: @johnthagen @markopy @valentijnscholten @elonzh @mblayman

[codecov]

Codecov Report

Merging #661 (6f1de26) into master (5da99c5) will increase coverage by 0.09%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master     #661      +/-   ##
==========================================
+ Coverage   98.80%   98.90%   +0.09%     
==========================================
  Files          63       63              
  Lines        7024     7025       +1     
==========================================
+ Hits         6940     6948       +8     
+ Misses         84       77       -7     

Impacted Files	Coverage Δ
drf_spectacular/contrib/django_filters.py	92.30% <92.00%> (+6.45%)	⬆️
tests/contrib/test_django_filters.py	100.00% <100.00%> (ø)

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