Add device_class to valve core config

[acshef]

What does this implement/fix?

This adds the device_class property to the valve core config. device_class is already implemented on the codegen side.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other

Pull request in esphome-docs with documentation (if applicable): N/A - documentation already mentions device_class

Test Environment

• ESP32
• ESP32 IDF
• ESP8266
• RP2040
• BK72xx
• RTL87xx

Example entry for config.yaml:

# Example config.yaml
valve:
  - platform: template
    id: water_valve
    device_class: water

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder). No tests have been added -- template valve already matches that of its nearest cousin: template cover.

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

• Documentation added/updated in esphome-docs. N/A - documentation already mentions device_class

[probot-esphome]

Hey there @esphome/core, mind taking a look at this pull request as it has been labeled with an integration (valve) you are listed as a code owner for? Thanks!
_{^{(message by CodeOwnersMention)}}

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.13%. Comparing base (4d8b5ed) to head (74245cd).
Report is 626 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #6765      +/-   ##
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+ Coverage   53.70%   54.13%   +0.42%     
==========================================
  Files          50       50              
  Lines        9408     9619     +211     
  Branches     1654     1698      +44     
==========================================
+ Hits         5053     5207     +154     
- Misses       4056     4086      +30     
- Partials      299      326      +27     

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