ot::Ncp::NcpBase::VendorGetPropertyHandler() method

Defines a vendor "get property handler" hook to process vendor spinel properties. The vendor handler should return `OT_ERROR_NOT_FOUND` status if it does not support "get" operation for the given property key. Otherwise, the vendor handler should behave like other property get handlers, i.e., it should retrieve the property value and then encode and write the value into the NCP buffer. If the "get" operation itself fails, handler should write a `LAST_STATUS` with the error status into the NCP buffer.