add function to create nip98 http authorization header

Closes: https://github.com/damus-io/damus/pull/1471
Signed-off-by: William Casarin <jb55@jb55.com>

damus/Nostr/Nip98HTTPAuth.swift

@@ -0,0 +1,23 @@
+//
+//  Nip98HTTPAuth.swift
+//  damus
+//
+//  Created by Fishcake on 2023/08/12.
+//
+
+import Foundation
+
+func create_nip98_signature (keypair: Keypair, method: String, url: URL) -> String? {
+    let tags = [
+        ["u", url.standardized.absoluteString], // Ensure that we standardise the URL before extracting string value.
+        ["method", method]
+    ]
+    
+    guard let ev = NostrEvent(content: "", keypair: keypair, kind: NostrKind.http_auth.rawValue, tags: tags) else {
+         return nil
+    }
+
+    let json = event_to_json(ev: ev)
+    let base64Header = base64_encode(Array(json.utf8))
+    return "Nostr " + base64Header // The returned value should be used in Authorization HTTP header
+}