Added a custom context parameter for the sync_to_async

[Arfey]

This pull request enhances the sync_to_async utility in asgiref/sync.py by allowing users to pass a custom contextvars.Context. This provides finer control over context variable propagation when running synchronous code in asynchronous environments. Comprehensive tests have been added to ensure correct behavior, especially regarding context isolation and parallel task execution.

More details https://code.djangoproject.com/ticket/36714#comment:17

[Arfey]

TypeError: create_task() got an unexpected keyword argument 'context'

[Arfey]

to keep the default behavior

import asyncio
import contextvars

value = contextvars.ContextVar('value')

async def main():
    value.set(0)
    
    async def inner():
        value.set(value.get() + 1)
    
    context = contextvars.copy_context()
    
    await asyncio.gather(
        asyncio.create_task(inner(), context=context),
        asyncio.create_task(inner(), context=context),
        asyncio.create_task(inner(), context=context),
    )
    
    assert context.get(value) == 3
    assert value.get() == 0
    
    print("Done")


asyncio.run(main())