Add domestic_hot_water_setpoint number to Plugwise

[bouwew]

Proposed change

Adding a new number: domestic_hot_water_setpoint, present on various OpenTherm boiler devices.

Info for HA devs: on some systems there is only a domestic_hot_water_setpoint-sensor present, to differentiate in the backend we renamed the relevant output-key to max_dhw_temperature. The number-name is "corrected" in strings.json.

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)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:

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
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• 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.
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To help with the load of incoming pull requests:

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

[home-assistant]

Hey there @CoMPaTech, @frenck, mind taking a look at this pull request as it has been labeled with an integration (plugwise) you are listed as a code owner for? Thanks!

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[bouwew]

@gjohansson-ST thanks for reviewing and merging the other two Plugwise PR's!

If you wouldn't mind, please review this PR too.

[gjohansson-ST]

Thanks @bouwew 👍
But don't ping for reviews unless asked

[bouwew]

Sorry if I was a bit too forward :)

Very much appreciated for the reviewing and merging!