Support for A2DP Bluetooth profile (mainly audio sender)

**Describe the problem you have/What new integration you would like**

No support for A2DP in ESPHome, I want it mainly as audio sender.

**Please describe your use case for this integration and alternatives you've tried:**

With the Bluetooth (proxy) support in ESPHome this ideally opens up the room for cheap multi-room audio streaming when ESP32s (or RP2040, I don't care which) can be connected to Bluetooth speakers (or headphones) via the A2DP profile.

I actually think the bluetooth proxy does not really add something. It's basically the ESP32 acting as mediaplayer for Hass and connecting over Bluetooth (A2DP) with speakers/headphones as "audio sink".

When I dream out loud, it would be extremely awesome to roam with a bluetooth headphone from room to room and have handovers between the ESPs... But I guess that's a few bridges too far :)

**Additional context**

I could find one github with an A2DP implementation for ESP32, maybe that can be used as a starting point? It both supports audio receiving & audio sending.
https://github.com/pschatzmann/ESP32-A2DP
However, this may be a bigger challenge than I thought: https://github.com/pschatzmann/ESP32-A2DP/wiki/WIFI-and-A2DP-Coexistence
Hope it still can be done, if needed with some additional (cheap) components.

Start by reviewing the linked ESP32-A2DP implementation and its Wi-Fi/A2DP coexistence notes. Then assess how audio-sender support could fit ESPHome's existing Bluetooth proxy and media-player behavior. Done would mean a defined implementation path for connecting an ESP32 to Bluetooth speakers or headphones and streaming audio.

Written by the indexing model from the issue text.