Add `playPromptUrl` for non-interrupting announcements / text-to-speech

[haigisto]

Motivation

Home Assistant's Text-to-Speech (TTS) integration uses the media_player.play_media action.
Using tts.speak stops the currently playing media during the announcement and does not resume it afterward.
TTS play_media actions contain the data attribute announce: true. This attribute currently has no effect on LinkPlay devices.
Please see Home Assistant's Media Player integration for further details.

Arylic LinkPlay devices provide the API endpoint playPromptUrl:

When this API is used, the device will lower current volume of playback and play the URL for notification sound.

Please see the API documentation for further details.

Test

I tested the endpoint on a Up2Stream Amp 2.0 V4 with firmware version 4.6.415145.36 using the following URL:

http://<amp_ip>/httpapi.asp?command=playPromptUrl:http://<file.mp3>

This worked as documented:
The music playback (vTuner / Spotify) continues at reduced volume and resumes normally after the notification file finishes playing.

Benefit

Using playPromptUrl:<url> for the previously described play_media action would enable non-interrupting announcements on Arylic LinkPlay devices.

If there is anything I can help with, please let me know.