Parameter references in Example Utterances can infer example Entity values

[dariowho]

AS A developer
I WANT TO write parameter references without specifying an example value
SO THAT I write examples faster and with less syntax

Details

Currently, an example utterance that contain a parameter must define an example entity value for it. For instance:

- My name is $user_name{John}

However, most of the times the example is not informative. We want to support writing the same utterance like:

- My name is $user_name

An example value is still needed when exporting the Agent to Dialogflow. Intents will have to draw an example from the Entity definition. Note that, while this is trivial for Custom Entities, it will require adding some examples to built-in System Entities.

Test Scenarios

TBD