ot::Ncp::NcpBase::VendorHandleFrameRemovedFromNcpBuffer() method

Is a callback which mirrors `NcpBase::HandleFrameRemovedFromNcpBuffer()`. It is called when a spinel frame is sent and removed from NCP buffer. (a) This can be used to track and verify that a vendor spinel frame response is delivered to the host (tracking the frame using its tag). (b) It indicates that NCP buffer space is now available (since a spinel frame is removed). This can be used to implement mechanisms to re-send a failed/pending response or an async spinel frame.