feat(sqla_factory): added __set_association_proxy__ attribute

[nisemenov]

Description

• an async context manager has been added in SQLAASyncPersistence class to prevent

...
raise RuntimeError('Event loop is closed')
RuntimeError: Event loop is closed

The garbage collector is trying to clean up non-checked-in connection <AdaptedConnection <asyncpg.connection.Connection
object>>, which will be terminated.  Please ensure that SQLAlchemy pooled connections are returned to the pool 
explicitly, either by calling ``close()`` or by using appropriate context managers to manage their lifecycle.

sys:1: SAWarning: The garbage collector is trying to clean up non-checked-in connection <AdaptedConnection 
<asyncpg.connection.Connection object>>, which will be terminated.  Please ensure that SQLAlchemy pooled connections are 
returned to the pool explicitly, either by calling ``close()`` or by using appropriate context managers to manage 
their lifecycle.

• build_async and batch_async methods have been added to work with Coroutine type field and to send them to the event pool;
• __set_association_proxy__ attribute has been added in SQLAlchemyFactory class to add AssociationProxy type fields in the result FieldMeta factory dict.

[github-actions]

Documentation preview will be available shortly at https://litestar-org.github.io/polyfactory-docs-preview/624

[adhtruong]

Is this behaviour generally used for other async methods or more specific to SQLA relationships? If specific to SQLA then should live within that factory

[nisemenov]

I should check a bit, but could suppose this is more specific for SQLAlchemy proxy. If this behaviour is only for the Alchemy, I'll try to think something and leave it within SQLAlchemy factory.

[adhtruong]

Nice. Adding as a feature makes sense here. Can appropriate tests and documentation be added for this feature please?

[nisemenov]

Thanks! Ok, I'd try to do that.