What does this implement/fix?

Now split-up config for a new water_heater component.

This requires more pull requests!

Order of PR:
1: water_heater api/core
2: water_heater component
3: water_heater template
4: water_heater web_server
5: This one

Types of changes

water_heater web_server

• 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)
• Developer breaking change (an API change that could break external components)
• Code quality improvements to existing code or addition of tests
• Other

Related issue or feature (if applicable):

• fixes: Support of water heaters feature-requests#2047)

Pull request in esphome-docs with documentation (if applicable):

• docs: Add water_heater component esphome-docs#5785
• HA: Add water_heater to esphome integration list home-assistant/home-assistant.io#42599
• HA-core: Add water_heater.py to ESPHOME home-assistant/core#159201

Test Environment

• ESP32
• ESP32 IDF
• ESP8266
• RP2040
• BK72xx
• RTL87xx
• nRF52840

Example entry for config.yaml:

water_heater:
  - platform: template
    id: my_boiler
    name: "Boiler"
    min_temperature: 10 °C
    max_temperature: 85 °C
    current_temperature: !lambda "return 45.0;"
    mode: !lambda "return water_heater::WATER_HEATER_MODE_ECO;"
    optimistic: true
    visual:
      min_temperature: 10 °C
      max_temperature: 85 °C
      target_temperature_step: 0.5 °C
      current_temperature_step: 0.1 °C

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

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

• Documentation added/updated in esphome-docs.