Implement serialisation and output DTOs

[hectorgomezv]

Dependencies

• Centralise schemas and data validation (TODO: link to issue)

Context here.

Summary
Currently, as shown in the context link, we have some inconsistencies between the current Client Gateway implementation responses and this new service ones. The idea here is, instead of doing ad hoc mapping in the services to the expected formats, having DTOs providing the expected payload format, and implement a serialisation layer which takes "service objects" and converts them to "presentation objects" to fulfil client's requirements.

Note
We could do some additional research here to see what's the best solution, but we're already using AJV to validate incoming payloads so we could use it for serialisation as well.

[hectorgomezv]

Blocked by #69

[fmrsabino]

Blocked by #69

More importantly. Do we need/want this. Even if the issue #69 is closed.