feat: add dart implementation for url signing

url_signer.dart

@@ -0,0 +1,30 @@
+import 'dart:convert';
+
+import 'package:crypto/crypto.dart'; // official dart package
+
+/// Sign a URL with a given crypto key
+/// Note that this URL must be properly URL-encoded
+String signUrl(String myUrlToSign, String privateKey) {
+  // parse the url
+  final url = Uri.parse(myUrlToSign);
+
+  final urlPartToSign = '${url.path}?${url.query}';
+
+  // Decode the private key into its binary format
+  final decodedKey = base64Url.decode(privateKey);
+
+  // Create a signature using the private key and the URL-encoded
+  // string using HMAC SHA1. This signature will be binary.
+  final bytes = utf8.encode(urlPartToSign);
+  final hmacSha1 = Hmac(sha1, decodedKey);
+  final digest = hmacSha1.convert(bytes);
+
+  final encodedSignature = base64Url.encode(digest.bytes);
+
+  return '$myUrlToSign&signature=$encodedSignature';
+}
+
+void main() {
+  final signedUrl = signUrl("http://maps.google.com/maps/api/geocode/json?address=New+York&sensor=false&client=clientID", 'vNIXE0xscrmjlyV-12Nj_BvUPaw=');
+  print(signedUrl);
+}