Added support for SynchronizedField on ManyToManyFields

[tognee]

This PR lets users set a ManyToManyField as a SynchronizedField without recieving the error:

Direct assignment to the forward side of a many-to-many set is prohibited. Use model_name.set() instead.

This does not solve the issue of whether a ManyToMany field should be translatable or syncronized, but gives the option for the user to syncronize it without errors.

This PR adds:

• the code changes to handle many to many fields
• a snippet model to test ManyToMany fields (with its migration)
• 3 unit tests to check if the SynchronizedField is respected correctly for ManyToMany

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.04%. Comparing base (8ba01e7) to head (fe388cf).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #891      +/-   ##
==========================================
+ Coverage   93.02%   93.04%   +0.01%     
==========================================
  Files          47       47              
  Lines        4245     4257      +12     
  Branches      551      552       +1     
==========================================
+ Hits         3949     3961      +12     
  Misses        177      177              
  Partials      119      119              

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[tognee]

I've seen that bitnami moved all free postgresql images to bitnamilegacy. I changed it just to test the checks, but I think it could it be replaced with the official docker postgresql image

[Marvinrose]

The codecov report flags one missing line in test/models.py, worth checking which line isn't covered and adding a test case for it.
The bitnamilegacy image change in test.yml makes sense as a short-term fix, but as you noted it might be worth replacing with the official postgres Docker image for long-term stability.

[tognee]

@Marvinrose I've aligned to main and added a test for the __str__ function in the test model
Now Codecov seems happy