Implement ExposedThing interface

[JKRhb]

The interface is already defined in the scripting_api library and an ExposedThing class exists in the core library, but it is currently missing implementations for the methods it should provide according to the API specification. One prerequisite for a working ExposedThing is also a ProtocolServer implementation which in our case should be a basic CoapServer.

Implementation steps

• Implement ExposedThing methods in core library.
• Test with an implemented CoapServer or rather a mocked ProtocolServer.