Add Thing and Attribute msg

[Rezenders]

Add a generic ROS messages to represent things and attributes.

Something like:

Attribute:

string name
rcl_interfaces/ParameterValue value

Thing:

string type
Attributes[] attributes

Example:

type: 'person'
attributes: [{
    'name': 'age',
    'value': {'type': 1, 'integer_value': 30 }
}]