Support value_template in MQTT triggers

[emontnemery]

Proposed change

Support value_template in MQTT triggers, to for example allow picking out a JSON key from the incoming MQTT message.

This is a follow up to #45614 which introduced templating of the payload and topic to match on.

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

Example entry for configuration.yaml:

# Example trigger configuration.
# The trigger will fire when receiving '{"wanted_key":"hello", "other_key":"abc"}'
- platform: "mqtt",
  topic: "test-topic",
  payload: "hello",
  value_template: "{{ value_json.wanted_key }}",

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request: Document value_template in MQTT triggers home-assistant.io#16754

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
• 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

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