Add base Subscribtion/recurring payments support

[PetrDlouhy]

Changes of Payment model, that are required to implement PayU backend and recurring payments.

I could implement these changes through some mixin mandatory only for PayU backend, but I think, that those methods might be common to more backends and it would be highly usefull to have same implementation for all backends. I am willing to rewrite them bit more general shape, if requested.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 75.00000% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 78.37%. Comparing base (8303e09) to head (20bdcf2). Report is 2 commits behind head on main.

:exclamation: Current head 20bdcf2 differs from pull request most recent head 4f9df42. Consider uploading reports for the commit 4f9df42 to get more accurate results

Files	Patch %	Lines
payments/models.py	73.07%	7 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #217      +/-   ##
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- Coverage   78.42%   78.37%   -0.05%     
==========================================
  Files          29       29              
  Lines        1979     2007      +28     
  Branches      244      244              
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+ Hits         1552     1573      +21     
- Misses        310      317       +7     
  Partials      117      117              

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[PetrDlouhy]

Hi @patrys, I rebased this to current master, and the methods are throwing NotImplementedError, if they can't figure out the variables from user.

Can you please give a new review? I think, it would be handy to unify these methods among providers.

[PetrDlouhy]

Here is sample implementation of the methods that connects django-payments to django-plans: https://github.com/PetrDlouhy/django-plans-payments/blob/feature/recurring-payments/plans_payments/models.py

[PetrDlouhy]

@WhyNotHugo @patrys Recently I was exploring PayPal subscription, which has completely different workflow (PayU payments are initiated from server, PayPal from the provider side).

I have reworked this completely based on your comments and also I tried to make this compatible with the provider initiated subscription, so it could be used for providers like PayPal.

This is still more proposal than done think, so I am open to suggestions how to make the whole recurring interface better.

[PetrDlouhy]

I have rebased this PR and fixed testing. I also added the requested JSONField.

@WhyNotHugo @patrys Could you please take a look? What needs to be done to make this merged?