Prefetch refactor

[oprypkhantc]

This PR targets two things:

• cleanup of #[Field], QueryFieldDescriptor and QueryField itself, separating prefetch functionality from them
• allow userland implementation of prefetch-like features, i.e. deferring the resolution of a field the same way prefetch does

This includes changes from #584, so that would need to be merged first; then this can be reviewed properly.

On the changes:

• prefetchMethod parameter of #[Field] now trigger a deprecation error
• new #[Prefetch] attribute supports callables, both through the container and through static methods: #[Prefetch([SomeService::class, 'method'])], #[Prefetch([SomeHelper::class, 'staticMethod'])], #[Prefetch('staticMethodInSourceClass')]
• non static calls on the source object are no longer supported: #[Prefetch('methodOnSource')]. There's an exception for the deprecated #[Field(prefetchMethod: 'method')] - it continues to work
• as a side effect of moving it to parameter level, multiple #[Prefetch] annotations per field are now allowed; order of input parameters in the scheme is determined by order of parameters on the field itself (see TestTypeWithPrefetchMethods)

[oojacoboo]

@oprypkhantc thanks for this. I'll get #584 reviewed and hopefully merged soon!

[oprypkhantc]

Hey @oojacoboo, this is ready for review.

[oojacoboo]

Thanks @oprypkhantc, looks good - merged