Add statistics websocket endpoint

[bramkragten]

Proposed change

Add websocket command to get statistics data.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

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• This PR fixes or closes issue: fixes #
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• Tests have been added to verify that the new code works.

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[probot-home-assistant]

Hey there @home-assistant/core, mind taking a look at this pull request as its been labeled with an integration (history) you are listed as a codeowner for? Thanks!
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[balloob]

Ok after test added.

[balloob]

How big will this data be? Should it use await connection.send_big_result(…) instead?

[balloob]

Looks good. Let's keep it like this for now, if we see problems we can change to using send_big_result.