nwc: add Nostr Wallet Connect logic

- WalletConnectURL parses nostrwalletconnect:// urls
- Add support for parsing and creating NWC requests

damus/Util/Keys.swift

@@ -10,7 +10,7 @@ import secp256k1
 
 let PUBKEY_HRP = "npub"
 
-struct FullKeypair {
+struct FullKeypair: Equatable {
     let pubkey: String
     let privkey: String
 }

damus/Util/WalletConnect.swift

@@ -0,0 +1,79 @@
+//
+//  WalletConnect.swift
+//  damus
+//
+//  Created by William Casarin on 2023-03-22.
+//
+
+import Foundation
+
+struct WalletConnectURL: Equatable {
+    static func == (lhs: WalletConnectURL, rhs: WalletConnectURL) -> Bool {
+        return lhs.keypair == rhs.keypair &&
+                lhs.pubkey == rhs.pubkey &&
+                lhs.relay == rhs.relay
+    }
+    
+    let relay: RelayURL
+    let keypair: FullKeypair
+    let pubkey: String
+    
+    func to_url() -> URL {
+        let urlstr = "nostrwalletconnect://\(pubkey)?relay=\(relay.id)&secret=\(keypair.privkey)"
+        return URL(string: urlstr)!
+    }
+    
+    init?(str: String) {
+        guard let url = URL(string: str), url.scheme == "nostrwalletconnect",
+              let pk = url.host, pk.utf8.count == 64,
+              let components = URLComponents(url: url, resolvingAgainstBaseURL: true),
+              let items = components.queryItems,
+              let relay = items.first(where: { qi in qi.name == "relay" })?.value,
+              let relay_url = RelayURL(relay),
+              let secret = items.first(where: { qi in qi.name == "secret" })?.value,
+              secret.utf8.count == 64,
+              let our_pk = privkey_to_pubkey(privkey: secret)
+        else {
+            return nil
+        }
+        
+        let keypair = FullKeypair(pubkey: our_pk, privkey: secret)
+        self = WalletConnectURL(pubkey: pk, relay: relay_url, keypair: keypair)
+    }
+    
+    init(pubkey: String, relay: RelayURL, keypair: FullKeypair) {
+        self.pubkey = pubkey
+        self.relay = relay
+        self.keypair = keypair
+    }
+}
+
+struct WalletRequest<T: Codable>: Codable {
+    let method: String
+    let params: T?
+}
+
+func make_wallet_pay_invoice_request(invoice: String) -> WalletRequest<PayInvoiceRequest> {
+    let data = PayInvoiceRequest(invoice: invoice)
+    return WalletRequest(method: "pay_invoice", params: data)
+}
+
+func make_wallet_balance_request() -> WalletRequest<EmptyRequest> {
+    return WalletRequest(method: "get_balance", params: nil)
+}
+
+struct EmptyRequest: Codable {
+}
+
+struct PayInvoiceRequest: Codable {
+    let invoice: String
+}
+
+func make_wallet_connect_request<T>(req: WalletRequest<T>, to_pk: String, keypair: FullKeypair) -> NostrEvent? {
+    let tags = [["p", to_pk]]
+    let created_at = Int64(Date().timeIntervalSince1970)
+    guard let content = encode_json(req) else {
+        return nil
+    }
+    return create_encrypted_event(content, to_pk: to_pk, tags: tags, keypair: keypair, created_at: created_at, kind: 23194)
+}