Improve the design of how we handle resource original source

**Motivation**
Right now, it is handled as plain json but we have to follow a certain set of rules about it, how we need to persist it, to extract an id, etc..

These rules should be formalized by introducing a dedicated type `Source` that would help us handle these operations and make it explicit that we are dealing with an original payload.

**Acceptance criteria**
1. The new type is introduced
2. The complexity of dealing with original source should be reduced and where we deal with it is better identified