provide `add_static_method_to_class` via `plugins.common`

[petergaultney]

Feature

Mypy's plugin helpers provide a few common cases, like adding an attribute and adding a method. I have a working implementation of adding a static method to a class, and as far as I can tell mypy does not currently provide a way of doing this.

Pitch

If a core developer had time to review a PR to introduce add_static_method_to_class to plugins.common, I'd be happy to spend some time figuring out how to contribute my code.

My current working version can be found here: https://github.com/xoeye/typecats/blob/main/typecats/cats_mypy_plugin.py?ts=4#L160

[thejaminator]

hey @AlexWaygood , @petergaultney , i had made a MR here. appreciate any comments #12221