Add yeelight nighlight support with separate light entity

[zewelor]

Description:

Added option to create separate light entity, for better nightlight mode switch UX. By default option is off, so its opt in.

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#<home-assistant.io PR number goes here>

Example entry for configuration.yaml (if applicable):

yeelight:
  devices:
    192.168.1.132:
      name: Wall light bedroom
      nightlight_switch_type: light

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

[github-actions]

Hey there @rytilahti, mind taking a look at this pull request as its been labeled with a integration (yeelight) you are listed as a codeowner for? Thanks!

[rytilahti]

This is waiting for #22547 and may require rebasing after that gets merged.

[balloob]

#22547 has been merged. Please rebase and address linting issues.