[Memo] All signed stuff should be timestamped

Not just ephemerals, but entities as well. Otherwise, one can do replay attacks!

I was reading through:

https://github.com/decentraland/katalyst/blob/dc2cb5fcdf21df34f639aac0b295a28232f19205/crypto/src/Authenticator.ts#L79

I think in this case it's fine because all Entities have a timestamp. But I wanted to leave this issue open as a reminder.