btif: expose get_wbs_supported method

This patch exposes the get_wbs_supported method so that floss
can query whether WBS is supported by the controller.

Bug: 244507283
Tag: #floss
Test: atest net_test_btif
Test: ./build.py --target test
Test: Manual test with next patch --
      "Floss: Add wbs support adapter API"

Change-Id: I5a6b5fbb0d6c5cd0d679983ab0cdaa2570b6b92f