Add start/stop VA services

[tetele]

This would enable users to automate triggering the VA pipeline externally, from HA. A few usecases:

• starting the VA when your face gets recognized by CompreFace
• pressing a button
• adding sentence triggers in automations to create conversations
• etc.

[andreasbrett]

I think this would benefit from checking if VA is already running and if the mute switch is turned off before starting VA (basically the conditions from the button click action).

[tetele]

@andreasbrett great idea! Thanks and done!

[jlpouffier]

I would propose to wait until this architecture discussion is built.
We're working on it as we speak: home-assistant/architecture#1143

The Assist Satellite entity will support starting a conversation natively.

I do not want to introduce something we know will be removed in the near future.

However I do agree with the need to start a pipeline remotely, but let's do it cleanly from the get-go

[tetele]

Historically, voice stuff and "near future" never went hand in hand. Also, I don't see any issue with making these service backwards compatible (e.g. by calling assist_satellite.start_conversation from the ESP side, albeit less efficiently), but I guess OK.

[balloob]

I agree with JLo, we're working on doing this a standardized way and we shouldn't complicate this.