Add type option "first" to sensor group in group component

[mpotthoff]

Proposed change

This PR adds a new type option called first to the sensor group in the group component.

A sensor group with type: first will always resolve to the first available sensor in that group.
By itself, that might not sound particularly useful, but when combined with the ignore_non_numeric configuration option, it becomes quite powerful - the combination will resolve to the first numeric (and thus available) sensor in the group.

My use case:

I control my heating using a temperature sensor in each room. Unfortunately, these sensors are battery-powered, so occasionally a battery runs out and the sensor becomes unavailable — usually when I’m away from home or on vacation. When that happens, my heating automations break because the room temperature is no longer available.

However, each room also has other sensors that additionally report the room temperature, just with lower accuracy.
This new feature allows me to combine all of those sensors into a single sensor group per room, ordered by precision. If the most accurate sensor becomes unavailable, the group automatically falls back to the next one. This way, my heating continues to function normally (just with less precision), and once I replace the battery, the group automatically switches back to using the first (most accurate) sensor.

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 #
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