Add custom loads function

[azimovMichael]

A PR for #2436 .
Added an option to add a custom loads function, the same way you can add a custom dumps.

[codecov]

Codecov Report

Merging #2445 (8ae3401) into main (13d5a44) will increase coverage by 0.047%.
The diff coverage is 83.333%.

@@              Coverage Diff              @@
##              main     #2445       +/-   ##
=============================================
+ Coverage   87.349%   87.396%   +0.047%     
=============================================
  Files           69        69               
  Lines         5549      5554        +5     
  Branches       964       966        +2     
=============================================
+ Hits          4847      4854        +7     
+ Misses         510       508        -2     
  Partials       192       192               

Impacted Files	Coverage Δ
sanic/request.py	95.287% <75.000%> (-0.227%)	⬇️
sanic/app.py	88.718% <100.000%> (+0.043%)	⬆️
sanic/server/protocols/http_protocol.py	83.823% <0.000%> (+2.205%)	⬆️

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