[le audio] Add new APIs to get assistant local metadata for source

getSourceMetadata is introduced to allow app to get sink's broadcast
source metadata.

Bug: 375421703
Bug: 375423982
Test: atest BluetoothLeBroadcastAssistantTest
Test: atest BassClientServiceTest
Test: manual test callback with LeAudioTestApp
Change-Id: I695bc0fe42bba991b5d895b548c0d00b966d3952