Add active_child_template to universal media player

[Drafteed]

Proposed change

The universal media player itself selects active child media player based on a sorted list of states (#68036), which may not always be suitable for a some tasks. This change adds a new parameter active_child_template that will allow to select (override) active child based on the template.

Basic example:

media_player:
  - platform: universal
    name: sony_tv
    children:
      - media_player.sony_tv_cast
      - media_player.sony_tv_psk
    active_child_template: >
      {% if is_state_attr('media_player.sony_tv_cast', 'app_name', 'TV') %}
         media_player.sony_tv_psk
      {% else %}
         media_player.sony_tv_cast
      {% endif %}

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)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request: Add active_child_template to universal media player home-assistant.io#26377

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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• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• For the updated dependencies - a link to the changelog, or at minimum a diff between library versions is added to the PR description.
• Untested files have been added to .coveragerc.

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[bieniu]

Looks good 👍

[emontnemery]

The name of the new parameter, active_children_template suggests plural, but the documentation suggests singular, i.e. a single active child:

Template must return the entity_id of the child that will be selected as active

[home-assistant]

Please take a look at the requested changes, and use the Ready for review button when you are done, thanks 👍

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[emontnemery]

LGTM, thanks @Drafteed 👍