iter states in states group

[Tishka17]

Description

This allows to iterate over all states in StatesGroup.
Aiogram already had in check for states group so this is relative feature.

Type of change

Please delete options that are not relevant.

• Documentation (typos, code examples or any documentation update)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• test_iterable

Test Configuration:

• Operating System: Ubuntu 20.04.2 LTS
• Python version: 3.8

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[JrooTJunior]

1. The PR description should be correctly filled
2. Patch-notes should be added to CHANGES directory
   For example execute command towncrier create 666.feature and then write short description of the changes to newly created file named CHANGES/666.feature

[codecov]

Codecov Report

Merging #666 (b28cabb) into dev-3.x (5851e32) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           dev-3.x      #666   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          269       269           
  Lines         5658      5660    +2     
=========================================
+ Hits          5658      5660    +2     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aiogram/dispatcher/fsm/state.py	100.00% <100.00%> (ø)