Add support for handling structure datatypes

In OPC UA servers often contain custom structure datatypes. This is especially true for PLCs, but also for other companion specifications.

The client therefor needs the ability to decode structures that are not known at compile time and needs a generic API for accessing those structure fields. Ther server provides the meta data for this datatypes via the UA information model.

The UACPP SDK provides the class UaGenericStructureValue for this usecase:

http://documentation.unified-automation.com/uasdkcpp/1.6.3/html/classUaGenericStructureValue.html