[Refactor] Notification plugins

[SchrodingersGat]

Breaking Change

This PR represents a breaking change to any custom plugin which implements notification functionality. To continue to function, any such plugin must be reworked to follow the new approach:

• Each custom notification method must be implemented as a separate plugin
• Each custom notification method must inherit the NotificationMixin class
• Refer to the 3x built-in notification plugins for examples

Details

This PR is a refactor of how notification methods are defined and collected.

Currently, it is a separate collection mechanism to plugins, and loading the notification methods is complex and somewhat "brittle" (compared to the plugin registry which has been optimized),

The intent here is to just make each notification method a plugin, which is then in line with our general concept of plugins. It also ensures that notification methods cannot be "collected" from plugins which are installed but not active (which is unfortunately the case currently).

TODO

• Split built-in notification methods into separate plugins
• Documentation for new built-in plugin types
• Reimplement solution for per-user-per-plugin settings
• Fix unit testing
• Check that email notifications still work
• Check that UI notification still work
• API unit tests for new plugin-user-setting
• Playwright tests for configuring plugin user settings

Related Issues

• [Bug] Transition Method Issues #9571

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[codecov]

Codecov Report

Attention: Patch coverage is 78.19905% with 46 lines in your changes missing coverage. Please review.

Project coverage is 85.25%. Comparing base (b02bc38) to head (54818f5).

Files with missing lines	Patch %	Lines
src/backend/InvenTree/plugin/api.py	52.17%	11 Missing ⚠️
...InvenTree/plugin/base/integration/SettingsMixin.py	58.33%	10 Missing ⚠️
src/backend/InvenTree/plugin/models.py	53.33%	7 Missing ⚠️
src/backend/InvenTree/common/notifications.py	78.57%	6 Missing ⚠️
...e/plugin/builtin/integration/core_notifications.py	92.45%	4 Missing ⚠️
src/backend/InvenTree/common/test_notifications.py	0.00%	3 Missing ⚠️
src/backend/InvenTree/InvenTree/permissions.py	66.66%	2 Missing ⚠️
src/backend/InvenTree/InvenTree/tasks.py	75.00%	1 Missing ⚠️
...nTree/plugin/base/integration/NotificationMixin.py	94.44%	1 Missing ⚠️
src/backend/InvenTree/plugin/base/label/label.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9735      +/-   ##
==========================================
- Coverage   86.30%   85.25%   -1.06%     
==========================================
  Files        1235     1238       +3     
  Lines       54258    54202      -56     
  Branches     2236     2236              
==========================================
- Hits        46828    46209     -619     
- Misses       6862     7425     +563     
  Partials      568      568              

Flag	Coverage Δ
backend	86.71% <78.19%> (-1.50%)	⬇️

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[matmair]

TBH there are so many lost / fundamentally changed mechanisms that I can not compare this to the current implementation as it just shares some entry points and names.
This means a new write of every even slightly complex notification method plugin and refactors for all plugins that trigger notifications.

[SchrodingersGat]

@matmair fair comments - I'll see if I can adjust this to reduce the number of changes required