Add Availability template to template sensor helpers

[dhover]

Describe your core improvement

In YAML you can define template sensors including a availability template. An availability template is used to suppress rendering of the state template if the source sensor does not have a valid numeric state. This is needed for e.g. energy sensors to supresss the spikes during the restart of Home Assistant. These spikes will also be shown in the energy dashboard.
By adding the availability template to the template sensor helper you can move all those yaml sensors to the template helpers in the GUI.

Current limitations

You have to define the sensors in YAML if you want to use the availability template in template sensors.

Technical benefits

No more spikes during restart of HA. No more YAML for these template sensors.

Additional context

No response

[Petro31]

Unfortunately, this is up to the core team. I would typically add this to the front end but it needs to be approved by core first. Now that we have collapsible sections, I may be able to add this without push-back.

[frenck]

So, I was watching GitHub when your response came in during my Core Architecture meeting.

We'd agreed on your proposal to allow for the availability option in a collapsed section.

So, you can go ahead directly without arch issue at this point.

../Frenck

[Petro31]

Excellent. Thanks @frenck. I'll put in a PR sometime soon :)

[epenet]

@Petro31 you might be able to re-use home-assistant/core#126807 / home-assistant/core#126806

[Petro31]

This PR adds availability to all template platforms in the UI.