Move used strucs on the factsengine to a entities package

[arbulu89]

Create a new entities package within factsengine to structure better the code and align with the new contracts repository entities. They still continue mirroring the contracts struct, but this are for internal consumption. Once they are done, they don't even need to have any json annotation, as they are not for messaging purposes.

• The facts_gathered file structs follow the same pattern than the contract (facts_gathered_v1.go
• The facts_gathering_request file structs are still the old ones, as the contract is not totally defined yet. Once it is, this must be changed

Besides that, a lot of copy/pasting and renaming