GD HCI: Support vendor specific event in GD

* Add a handler for vendor specific command
* Add registration and unregistration APIs
* LOG_ERROR when an unknown vendor specific event is received
* Add unit tests to validate the above functions

Tag: #gd-refactor
Bug: 183057550
Test: bluetooth_test_gd
Change-Id: I00d6d892f3d60a8909352061dcae45f92d47b91d