Introduce BTM_BleOpportunisticObserve

Register/unregister opportunistic scan callback. This method does not trigger scan start/stop, but if scan is ever started, this callback would get called with scan results. Additionally, this callback is not reset on each scan start/stop. It's intended to be used by LE Audio related profiles, that would find yet unpaired members of CSIS set, or broadcasts.

Bug: 150670922
Tag: #feature
Test: compile & manual testing
Sponsor: jpawlowski@
Change-Id: Ia012b08a29addfa5de16ef692114472d75efda8b