Add NIP 9a for push notifications

9a.md

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+NIP-9a
+======
+
+Nostr Relay Push
+----------------
+
+`draft` `optional`
+
+This NIP defines a way for users to ask relays to push events to other servers.
+
+To register a push subscription, users may create a `kind 30390` event. This event defines the following tags:
+
+- `d` - a random string
+- `relay` - a nostr relay URL to monitor (may appear multiple times)
+- `filter` - a nostr filter for matching events (may appear multiple times)
+- `ignore` - an nostr filter for ignoring matched events (optional, may appear multiple times)
+- `callback` - a URL indicating a callback url to forward events to
+- `include_event` - if included, relays MUST agree to include the complete `event` data in the push payload or reject the subscription
+
+If a relay does not intend to fulfill the subscription, it SHOULD respond with an `OK` message with `false` as the result and a human-readable message.
+
+When a relay receives a new event, it SHOULD match it against all current registrations and forward the event to authorized users by sending a `POST` request to the `callback` URL with `Content-Type: application/json`. The payload should be as follows:
+
+```typescript
+{
+  id: string          // The event's id
+  relay: string       // The normalized url of the relay sending the notification
+  event?: Event       // The event itself, if the subscription specifies `include_event`
+}
+```
+
+If the `callback` URL responds with a `404` status code, the relay SHOULD delete the subscription.
+
+## Miscellaneous recommendations
+
+- Relays MAY delete subscriptions at their discretion (e.g. due to expiration, inactivity, callback `500` errors, etc).
+- Clients MAY delete subscription events as specified in [NIP 09](09.md).
+- Clients SHOULD re-sync subscription events periodically to ensure they continue receiving push notifications.
+- If a relay receives a `kind 30390` event whose `relay` tag does not match the relay's own URL, the relay SHOULD discard it.
+- Relays that implement this NIP MUST include `9a` in its [NIP 11](11.md) `supported_nips`.
+- Relays MUST serve `kind 30390` events only to their author.
+- Clients MUST NOT send `kind 30390` events to relays that do not advertise support for this NIP.
+
+## Example: Push Notifications
+
+This NIP may be used to facilitate push notifications sent over FCM, APN, VAPID, or other types of server. To do so, a client developer can register device tokens with their own server, returning a `callback` to the client. The client can then construct `kind 30390` events signed by the user containing the `callback` to relays they want to get notifications from:
+
+```typescript
+{
+  kind: 30390,
+  pubkey: "<user pubkey>",
+  content: "",
+  tags: [
+    ["d", "<subscription id>"],
+    ["relay", "wss://relay.example.com/"],
+    ["filter", "{\"kinds\": [1], \"#p\": [\"<user pubkey>\"]}"],
+    ["ignore", "{\"#t\": [\"footstr\"]}"],
+    ["callback", "https://my-callback.example.com/<subscription-id>"]
+  ],
+}
+```
+
+When the subscription is no longer needed, clients can delete subscription events or cancel the subscription with the push server directly, relying on the server to return a 404 to relays sending events.