Add timestamp with historical values (missing membership history)

In transfer we have blockNumber as well as timestamp:

type Transfer @entity {
  blockNumber: Int! @index
  timestamp: DateTime! @index

This is convenient because:

• most UI will display timestamp to the user instead of blocknumber, so this avoids fetching time from blocknumber (but this is redundant)
• it allows to have the real v1 timestamp even when the blocknumber is 0 (genesis)

We could extend this to other places where there is a blocknumber to make migration more "smooth" for the users. This would for example allow to see the v1 history of membership.

In order to do this, we will have to add:

• membership_history.json (missing)
• certification_history.json
• transaction_history.json

at the output of py-g1-migrator. This is a bit of work but is linked to the forum topic https://forum.duniter.org/t/v2s-smooth-gradual-migration-clients-and-indexers-side/9827.