Add "Nest Audio" device

[Molkobain]

Objective
This PR adds the "Nest Audio" device to the GCast devices list so its picture can be displayed correctly.

Current behavior
As you can see below, currently it displays the default model picture:

Expected behavior
Here is the result with the device correctly displayed

Cause
This is due to the model name being "Nest Audio"

Proposed solution
Unfortunately, unlike for "Google Nest Mini" or "Google Nest Hub", the device's model isn't prefixed with "Google " which makes it less homogenous. So I used the "real" model name in the GCAST_MODELS key but kept the picture name homogenous with the other Nest devices.

        'nest audio' =>
            array('logo' => 'model_googlenest_audio.png', 'ga' => true),

[Molkobain]

Hello @guirem any update on the PR? :)