Send a gift wrap signed by custom signer to NIP-17 relays.

[reyamir]

Describe the enhancement
In my app, I need to send a gift wrap signed by custom signer to user's nip17 relays. But there are no functions allow to do it yet.

I've made a custom function like this in my fork. But I'm not sure about it so I didn't created a PR.

/// Send the gift wrap event to relays
///
/// # Overview
///
/// Send the [`Gift Wrap`] to all relays with [`RelayServiceFlags::WRITE`] flag.
///
/// # Gossip
///
/// If `gossip` is enabled (see [`Options::gossip`]):
/// - the [`Gift Wrap`] will be sent also to NIP17 relays (automatically discovered);
#[inline]
#[cfg(feature = "nip59")]
pub async fn send_gift_wrap(&self, gift_wrap: &Event) -> Result<Output<EventId>, Error> {
    // NOT gossip, send event to all relays
    if !self.opts.gossip {
        return Ok(self.pool.send_event(gift_wrap).await?);
    }

    // Send event using gossip
    self.gossip_send_event(gift_wrap, true).await
}

Use case
Useful for NIP-4E

[yukibtc]

The issue with this is that will allow to send non-NIP17 events to NIP17 inbox. If the behavior is correct (is ok to send any NIP59 event to relays in 10050 list) I can add it.

Otherwise I prefer to find another solution. Like, what if for now I expose the gossip_send_event as private API? This would allow you to send events with gossip and choose if send them as NIP17 or not, but will not show it in public docs and APIs.

[reyamir]

I think exposed gossip_send_event is simplest solution.

[yukibtc]

@reyamir, for now I've exposed the __gossip_send_event as private API at commit cd5f12c. Is not merged in master branch yet, I hope to find another solution before the next release.