feat: allow extra filtering fields

[enkhjile]

Hello, I'm the guy who opened #600. ~~If you think that the implementation is good enough, I will proceed to update the documentation accordingly.~~

Edit: I've updated the docs

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (7dd2f2c) to head (0fd8574).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #619   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            8         8           
  Lines          237       239    +2     
=========================================
+ Hits           237       239    +2     

Files with missing lines	Coverage Δ
fastapi_filter/base/filter.py	100.00% <100.00%> (ø)

[enkhjile]

@arthurio Hello, can you review this? (Somehow test workflow on python 3.9 failed during codecov upload)