nostr: allow disabling timestamp tweaking for NIP-59

[yukibtc]

Allow disabling timestamp tweaking for NIP-59 seals (and gift-wraps?).

https://primal.net/e/nevent1qqsf377pna8vh3f3yxjfy2n4usqlpa0vzf4439ap84nswsevrktaj7s958fzl

[TheAwiteb]

I think let the user provide a custom time would be better, the they can disable it