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
Merged-In: Id5f5afc0130c5642d6cad8098ac8502783c15621
(cherry picked from commit 4fd1cd0a)