refactor: fix proxy system and introduce psalm extension

[nikophil]

this PR partially fixes the issue https://github.com/zenstruck/foundry/issues/696:

I've decided to completely remove all methods in PersistenceProxyObjectFactory, so we do not override @final methods anymore. We still need to figure out a solution for PersistenceObjectFactory::create(), I'll do this in a further PR.

fixes https://github.com/zenstruck/foundry/issues/701:

Psalm does not understand the following:

/**
 * @extends PersistentProxyObjectFactory<User>
 */
class UserFactory extends PersistentProxyObjectFactory{}

/**
 * @template T of object
 * @extends PersistentObjectFactory<T&Proxy<T>>
 */
abstract class PersistentProxyObjectFactory extends PersistentObjectFactory{}

/**
 * @template T of object
 */
abstract class PersistentObjectFactory{}

it always consider the template to be User and not User&Proxy<User>. I think because we're breaking covariance (T&Proxy<T> is wider than T)

I've finally decided to introduce a psalm extension, we will be able to fix nearly every typing problem in here.

[nikophil]

I'm unfamiliar with psalm plugins, will this be auto-registered in projects using foundry and psalm?

yes, thanks to the change in composer.json