Add a syntax for merging lists of triggers

[karwosts]

Proposed change

The user of Trigger Selectors in blueprints currently has a significant limitation in that the selector returns a list of triggers, but there is no yaml syntax to merge a list of triggers from the selector with a list of triggers hardcoded in the blueprint.

So if a blueprint wants to trigger on some predefined behavior, it cannot allow user to configure additional triggers with a selector because the lists cannot be merged.

To mitigate this, I propose a syntax (under platform: trigger) for triggers which will allow for combining a list of triggers from an input with a hardcoded list of triggers. When the backend encounters this list, it will just be flattened into a single list of triggers.

E.g.:

blueprint:
  name: Nested Trigger Blueprint
  domain: automation
  input:
    usertrigger:
      selector:
        trigger:

trigger:
  - platform: time_pattern
    seconds: /10
  - platform: trigger
    trigger: !input usertrigger
condition: []
action: []

This can trigger the blueprint on the predefined time pattern, in addition to any triggers specified by the user.

I'm sure there's still more work to be done here with testing, but wanted to see if this idea has merit. It worked for me as a basic proof of concept.

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 Blueprint: cannot add triggers from "trigger:" selector to trigger section when static triggers are already set. #106706
  fixes Trigger selector: cannot add one from selector if one already exists in the blueprint frontend#19050

• This PR is related to issue:
  https://community.home-assistant.io/t/merge-trigger-selector-with-other-triggers/655399
  https://community.home-assistant.io/t/help-using-the-new-trigger-selector-2023-12-in-blueprint/653107

• Link to documentation pull request:

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