Give user registered types priority when encoding / decoding JSON

[soceanainn]

User registered types should take priority over default types in Kombu.

Although users can currently override the encoder/decoder for a Kombu registered type by calling register_type (for example calling register_type(Decimal, ...) in their own code), this doesn't work well when it comes to subclassing. If users currently need to pass subclasses of any Celery registered types (datetime, date, time, Decimal or UUID) they would be forced to either:

1. Override implementation of superclass with a subclass aware encoder / decoder implementation, or
2. Access the 'protected' _encoders dictionary in the Kombu json module, pop the value for the superclass, add their subclass, and then re-add the superclass.

By separating user registered types and Kombu registered types into separate dictionaries, we can always give priority to user registered types instead, which simplifies this process for users (although technically it is a breaking change from existing behaviour).

Solves #1895

[codecov]

Codecov Report

Attention: Patch coverage is 95.65217% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.50%. Comparing base (6c00d9a) to head (2066474).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
kombu/utils/json.py	95.65%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2188      +/-   ##
==========================================
+ Coverage   81.49%   81.50%   +0.01%     
==========================================
  Files          77       77              
  Lines        9509     9520      +11     
  Branches     1148     1153       +5     
==========================================
+ Hits         7749     7759      +10     
  Misses       1569     1569              
- Partials      191      192       +1     

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[thedrow]

@soceanainn Can you please check why the tests are failing before I review?