Implement ExposedThing interface

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`.

Start by reading the ExposedThing interface in the scripting_api library and its existing class in the core library, then review the API specification for the required methods. Use a mocked ProtocolServer or a basic CoapServer to exercise the implementation; done means the methods are implemented and tested against that server boundary.

Written by the indexing model from the issue text.