Allow setting device_id to main device on sensors

[danez]

Describe your core improvement

It would be great if I could set a device_id explicitly to the main device.

Current limitations

Right now it can be only set to sub-devices or be omited. But omitting is not always possible in yaml, e.g. in shared packages.

Technical benefits

In a shared package it would open up the possibility to assign device_id conditionally based on e.g. vars.

Example (assuming leaving device_id an empty string means main device):

defaults:
  device_id: ""

sensors:
  - device_id: "${device_id}"

or other option (where "main" is always the id of the main device):

defaults:
  device_id: "main"

sensors:
  - device_id: "${device_id}"

Additional context

I have some shared packages (1) which i would love to use the new sub-device feature. But i would like to make it optional.

So i would love my package to have a var device_id which can be set to assign all sensors in the package to a sub device. If the var is omitted, it should be assigned to the main device.

This is currently not possbile, because i cannot conditionally add the device_id field in yaml.

(1) https://github.com/danez/esphome-zendure-bridge/blob/v1/packages/hub2000.yaml

[jesserockz]

esphome/esphome#10217