Extending subscription support

[sdobbelaere]

As promised on https://github.com/strawberry-graphql/strawberry-graphql-django/issues/454 I'm hereby publishing my subscription implementation for OneSila. .

What does it do:

• Allow you to expose every model to the frontend via subscriptions.
• Easy to extend and modify.

What it doesn't do:

• Support query and mutation extensions
• Follow any other internal lib safety features or restrictions.

This PR is intended as first step towards comprehensive subscriptions support for models out of the box. The reason this wasn't published sooner is because this PR should be stamped as experimental.

Types of Changes

• [x] Core
• [ ] Bugfix
• [x] New feature
• [x] Enhancement/optimization
• [x] Documentation

Issues Fixed or Closed by This PR

• https://github.com/strawberry-graphql/strawberry-graphql-django/issues/454

Todos:

• [ ] Investigate query/mutation extension support
• [ ] Create one liner field declarations
• [ ] More flexible access controle
• [ ] Improve extendability
• [ ] Tests
• [ ] Add receivers import into the app.py file

Want to help?

• Test need figuring out as this is Django-channels based.
• ACL and extensions are an important part of query/mutations. This probably needs investigating.
• model_subscriber() wrapper is a not a great approach towards extendability.
• ...

[codecov-commenter]

Codecov Report

Attention: 101 lines in your changes are missing coverage. Please review.

Comparison is base (b5adb23) 87.89% compared to head (0ce4729) 85.18%.

Files	Patch %	Lines
strawberry_django/subscriptions/publishers.py	0.00%	69 Missing :warning:
strawberry_django/subscriptions/subscribers.py	0.00%	9 Missing :warning:
strawberry_django/receivers.py	0.00%	8 Missing :warning:
strawberry_django/subscriptions/receivers.py	0.00%	8 Missing :warning:
strawberry_django/subscriptions/utils.py	0.00%	7 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #467 +/- ## ========================================== - Coverage 87.89% 85.18% -2.71% ========================================== Files 37 42 +5 Lines 3164 3267 +103 ========================================== + Hits 2781 2783 +2 - Misses 383 484 +101 ```

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