LeAudio: Introduce codec wrapper interface

This extracts the encoding logic from client.cc,
for better code separation and maintainance.

Bug: 295972694
Bug: 262481440
Bug: 262481609
Test: atest bluetooth_le_audio_client_test bluetooth_le_audio_test bluetooth_test_broadcaster_state_machine bluetooth_test_broadcaster
Change-Id: Iee2d1a53c0a0955a8ff7bdd0e9e705ad17159cd9