le_audio: Implement voice back channel support

This patch introduces support for high quality output with input
possibility scenario for LE Audio under voice assistant context.

Test: atest bluetooth_le_audio_client_test
Sponsor: @jpawlowski
Bug: 224933710
Change-Id: Iade6749a158a1dcb92a574e2c011565617c61d8e
(cherry picked from commit 68d64d3d6ca8dc8554e8e6026fdb9715826f7a11)