Bugfix evohome

[zxdavb]

Breaking Change:

Device state attributes keys have been changed to snake_case and, since this a breaking change, the opportunity was taken to revise/simplify the entire device state attributes schema.

Description:

evohome is a complicated integration, has had major changes in recent times, and is not yet mature - this PR addresses some bugs and related architectural issues.

• Modify behavior of set_temperature to mirror that of the vendor's UI (e.g. any target temp changes until next setpoint, unless explicitly set otherwise)
• setpoint datetimes are now correctly converted to local/aware isoformat (internally, and for logging/state attributes)
• use snake_case for device state attributes
• add a unique_id for each entity (systemId, zoneID, dhwId)
• refactoring - remove API wrappers (no longer using private methods)
• add away mode for DHW (set to permanent off or Auto, depending)

Related issue (if applicable): fixes #25985, #25984 (both these, and the above, are related)

Pull request with documentation for home-assistant.io (if applicable): N/A

Example entry for configuration.yaml (if applicable): N/A

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.
• I have followed the development checklist

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

• Documentation added/updated in home-assistant.io

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

• The manifest file has all fields filled out correctly. Update and include derived files by running python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt by running python3 -m script.gen_requirements_all.
• Untested files have been added to .coveragerc.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[zxdavb]

@balloob do you have time for another look?