le_audio: Add single microphone scenario for conversational context

Some devices may be microphone only audio devices, allow them to be
usable in LE Audio context.

Test: atest bluetooth_le_audio_client_test
Sponsor: @jpawlowski
Bug: 229802353
Change-Id: Id5f5afc0130c5642d6cad8098ac8502783c15621
(cherry picked from commit e32b43e20549a549f659ea81432d805d6b7d69de)