Add power service to System Bridge integration

[timmo001]

Proposed change

Adds new power command service to System Bridge integration.

Continution of #72867 addressing review.

Type of change

• New feature (which adds functionality to an existing integration)

Additional information

• Link to documentation pull request: Add power command to System Bridge docs home-assistant.io#28037

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• I have followed the perfect PR recommendations
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

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

• Documentation added/updated for www.home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• For the updated dependencies - a link to the changelog, or at minimum a diff between library versions is added to the PR description.
• Untested files have been added to .coveragerc.

To help with the load of incoming pull requests:

• I have reviewed two other open pull requests in this repository.

[BenoitAnastay]

Seem alright and it does fix the previous style issue from #72867 (comment)

Thus I gues it can be merged

[home-assistant]

Please take a look at the requested changes, and use the Ready for review button when you are done, thanks 👍

Learn more about our pull request process.

[joostlek]

LGTM!