feat: add payment_sent notification

[im-adithya]

Fixes getAlby/hub#64 | Adds support for payment_sent notification

• Have to add this to the NIP-47 docs, waiting for confirmation on the notification payload
• js-sdk PR: feat: add payment_sent notification js-sdk#223

Testing

• Added test in NWC
• split tests - @im-adithya
• Check from rails app - @im-adithya
• Update NIP-47 notifications PR - @rolznz - TODO afterwards.
• Wait for response from Kukks to see we don't break something - @rolznz - yes, it will just fall back to polling (but I don't think that's the case anyway because it probably just uses the published info event)

[im-adithya]

To be added in nostr-protocol/nips#1164

### `payment_sent`

Description: A payment was successfully sent from the wallet.

Notification:
```jsonc
{
    "notification_type": "payment_sent",
    "notification": {
        "type": "outgoing",
        "invoice": "string", // encoded invoice
        "description": "string", // invoice's description, optional
        "description_hash": "string", // invoice's description hash, optional
        "preimage": "string", // payment's preimage
        "payment_hash": "string", // Payment hash for the payment
        "amount": 123, // value in msats
        "fees_paid": 123, // value in msats
        "created_at": unixtimestamp, // invoice/payment creation time
        "expires_at": unixtimestamp, // invoice expiration time, optional if not applicable
        "settled_at": unixtimestamp, // invoice/payment settlement time
        "metadata": {} // generic metadata that can be used to add things like zap/boostagram details for a payer name/comment/etc.
    }
}

[rolznz]

@im-adithya can you update this PR?

[im-adithya]

Done!

[rolznz]

@im-adithya @bumi I looked at the rails code and it seems to look ok to handle both incoming and outgoing invoices without any changes

I'm merging this now. In case you have any feedback, let me know and I'll address it in a follow up PR