Add state_list to StatesGroup

[Cub11k]

Description

Include changes, new features and etc:

Describe your tests

How did you test your change?

Python version:

OS:

Checklist:

• I added/edited example on new feature/change (if exists)
• My changes won't break backward compatibility
• I made changes both for sync and async

[coder2020official]

Why not to add it during the initialisation of the subclass and then return the list?

[Cub11k]

Definitely better, agreed