Add add_overloaded_method_to_class helper to plugins/common.py

[sobolevn]

There are several changes:

1. add_overloaded_method_to_class itself. It is very useful for plugin authors, because right now it is quite easy to add a regular method, but it is very hard to add a method with @overloads. I don't think that user must face all the chalenges that I've covered in this method. Moreover, it is quite easy even for experienced developers to forget some flags / props / etc (I am pretty sure that I might forgot something in the implementation)
2. add_overloaded_method_to_class and add_method_to_class now return added nodes, it is also helpful if you want to do something with this node in your plugin after it is created
3. I've refactored how add_method_to_class works and reused its parts in the new method as well
4. tvar_def in add_method_to_class can now accept a list of type vars, not just one

Notice that add_method_to_class is unchanged from the user's POV, it should continue to work as before.

Tests are also updated to check that our overloads are correct.

Things to do later (in the next PRs / releases):

1. We can possibly add is_final param to methods as well
2. We can also support @property in a separate method at some point

[sobolevn]

GitHub is down 😒

[github-actions]

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