gd/hci: handle MSFT_LE_MONITOR_DEVICE_EVENT in gd layer

Allow MsftExtensionManager to accept `ScanningCallback` to be
registered, which will be triggered on MSFT scan events (e.g. device
found and lost).

Bug: 261482382
Tag: #floss
Test: ./build.py --target test
Test: Manual test on a ChromeOS device, e.g., zork/morphius
      localhost # btclient
      bluetooth> le-scan register-scanner
      bluetooth> le-scan start-scan 1
      # If a target LE device is found and then lost, the log shows
      # something like
      bluetooth> btclient:info: Scan result lost: ScanResult {
        name: ...,
        address: ...,
        ...
      }

Change-Id: Ie255ef5c7d058a621e83c0eed241916de29a51a8