Keep volume control enabled for media_player entities where 'off' is more like 'standby'

Feature request to optionally allow the volume control (attribute volume_level, as well as functions to read and set volume in the default GUI) for media_player entities even when in ‘off’ state.

I have plenty of Google Home speakers, such as the o.g. Home Minis, Nest Minis, Chromecast Audios, and 3rd party speakers. Even when these are in state ‘off’, you can still adjust their volume in Home Assistant. For most default GUI elements in Home Assistant, when a media_player is in state ‘off’, the current volume level is unavailable and it looks like volume control is disabled. However, you can still set the volume through the volume_set action, or non-default GUI elements, even if they are not able to read it back.