Add json_attributes_template

[emontnemery]

Description:

Add json_attributes_template to give a little bit of flexibility to json_attributes_topic, which has been requested several times, most recently in #22924.

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#9332

Example entry for configuration.yaml (if applicable):

sensor:
  - platform: mqtt
    name: "Timer 1"
    state_topic: "tele/sonoff/sensor"
    value_template: "{{ value_json.Timer1.Arm }}"
    json_attributes_topic: "tele/sonoff/sensor"
    json_attributes_template: "{{ value_json.Timer1 | tojson }}"

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.

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

• Documentation added/updated in home-assistant.io TODO

If the code does not interact with devices:

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

[github-actions]

Hey there @home-assistant/core, mind taking a look at this pull request as its been labeled with a integration (mqtt) you are listed as a codeowner for? Thanks!

[codecov]

Codecov Report

Merging #22981 into dev will decrease coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev   #22981      +/-   ##
==========================================
- Coverage   94.11%   93.98%   -0.13%     
==========================================
  Files         448      449       +1     
  Lines       36718    36765      +47     
==========================================
- Hits        34558    34555       -3     
- Misses       2160     2210      +50

Impacted Files	Coverage Δ
homeassistant/helpers/service.py	93.19% <0%> (-0.08%)	⬇️
homeassistant/helpers/entity_component.py	96.18% <0%> (-0.03%)	⬇️
homeassistant/components/demo/media_player.py	95.42% <0%> (ø)	⬆️
homeassistant/components/esphome/camera.py	0% <0%> (ø)

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[balloob]

This won't help the issue since json.dumps is not available in a template?

[emontnemery]

@balloob not json.dumps exactly, but filter tojson is available , so a template like this should be fine: 'json_attributes_template': "{{ value_json['Timer1'] | tojson }}"