Add creation timestamp to Operation

[aoudiamoncef]

Add creation timestamp to Operation.

Describe the solution you'd like Add a field created_at to Operation type

Additional context Seen with innovation team

[damip]

This has very deep consequences on the architecture. We can't just add this field to the operation structure. However, we can somehow register the timestamp at which a given node has seen a given operation for the first time, and have the API return it on demand