JSON encoder change via app

[ahopkins]

Resolves #2054

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This one has come up a few times in the past. The current method for setting this at the default level by only looking at the environment is not ideal.

Adds the following API to set the encoder globally.

Sanic(..., dumps=my_favorite_json_encoder)

[codecov]

Codecov Report

Merging #2055 (9039319) into master (d76925c) will increase coverage by 0.012%.
The diff coverage is 100.000%.

@@              Coverage Diff              @@
##            master     #2055       +/-   ##
=============================================
+ Coverage   91.995%   92.007%   +0.012%     
=============================================
  Files           36        36               
  Lines         3298      3303        +5     
  Branches       562       564        +2     
=============================================
+ Hits          3034      3039        +5     
  Misses         179       179               
  Partials        85        85               

Impacted Files	Coverage Δ
sanic/blueprint_group.py	100.000% <ø> (ø)
sanic/app.py	91.525% <100.000%> (+0.036%)	⬆️
sanic/blueprints.py	100.000% <100.000%> (ø)
sanic/models/futures.py	100.000% <100.000%> (ø)
sanic/response.py	94.737% <100.000%> (+0.106%)	⬆️

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[harshanarayana]

Looks good to me. Thanks for taking care of some of the import sorting issues.

[ahopkins]

Thanks isort 🤘