feat: let ProxyMethod resolve callable attribute targets

[yeetypete]

Summary

Let ProxyMethod resolve a target declared as a class attribute, in addition to an ordinary instance method. A target is accepted whenever pyrefly can call it: Callable[...] annotations, callback protocols, and type[X] constructors all work but non-callable targets such as forward: int are still rejected.

This lets the shape stubs declare nn.Module.forward as Callable[..., Any], matching PyTorch:

https://github.com/pytorch/pytorch/blob/d6c03540dc0f040e1d8a920d19d9333de6513545/torch/nn/modules/module.py#L526

The stubs previously declared forward as a method because that was the only target ProxyMethod could resolve. Under strict-callable-subtyping, a method-form forward(*args, **kwargs) makes every concrete forward(self, x) override a bad-override.

Test Plan

Run test.py.

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D119805915. (Because this pull request was imported automatically, there will not be any future comments.)

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅