`gravatar`- Update date/time fields

[hamorillo]

Closes #157

Description

We'll need to update api-gravatar.json with the final version of the definition. But in order to keep moving, updating it manually based on previous discussion.

On the other hand, Instant class is a better representation of the DateTime format in our SDK. So, we are modifying the mapping configuration.

We also need to add an adapter to deserialize and string with the proper format to an Instant instance.

Testing Steps

• Smoke test the demoApp, code review, and CI should be enough. (The change will be more important with a following PR that will enable the API Key so we'll receive those values).