feat(cryptographic): add apiKey API method

lib/src/cryptographic.dart

@@ -141,4 +141,29 @@ class Cryptographic {
 
     return '$header64.$payload64.$signature';
   }
+
+  /// Returns API key.
+  ///
+  /// [prefix] is optional prefix string (e.g. `sk_`, `pk_`, `api_`).
+  /// Default is no prefix.
+  ///
+  /// [length] is optiontal legnth of the random part (default is 32).
+  ///
+  /// [asHex] determines whether returned string should be in hex or base64
+  /// (default is hex).
+  ///
+  /// Example:
+  /// ```dart
+  /// Cryptographic().apiKey(); // "f59e6e027fd23b6721cfa79eb28f58ae"
+  /// Cryptographic().apiKey(prefix: 'sk_'); // "sk_e95cabd07b0ac9222799ecd4937e6137"
+  /// Cryptographic().apiKey(prefix: 'pk_', length: 48, asHex: false); // "pk_F7gwa47ywr3yi6UDG-El4sqNUCN_maLAcjZY5omAONoa2WBR"
+  /// ```
+  String apiKey({String prefix = '', int length = 32, bool asHex = true}) {
+    final key = asHex
+        ? tokenHex(entropy: length ~/ 2)
+        : tokenUrlSafe(
+            entropy: length,
+          ).replaceAll('=', '').substring(0, length);
+    return '$prefix$key';
+  }
 }

test/src/cryptographic_test.dart

@@ -156,5 +156,41 @@ void main() {
         );
       }
     });
+
+    test('returns api key with default params', () {
+      final result = crypto.apiKey();
+      expect(result.length, equals(32));
+      expect(seededCrypto.apiKey(), equals(seededCrypto.apiKey()));
+    });
+
+    test('returns api key with provided params', () {
+      const params = [
+        // [prefix, length, asHex, expectedMinLength]
+        ('', 32, true, 32),
+        ('sk_', 32, true, 35),
+        ('pk_', 32, true, 35),
+        ('api_', 32, true, 36),
+        ('', 64, true, 64),
+        ('test_', 16, true, 21),
+        ('', 32, false, 32),
+        ('sk_', 32, false, 35),
+        ('pk_', 48, false, 51),
+      ];
+
+      for (final (prefix, length, asHex, expectedMinLength) in params) {
+        final result = crypto.apiKey(
+          prefix: prefix,
+          length: length,
+          asHex: asHex,
+        );
+        expect(result.length, equals(expectedMinLength));
+        expect(
+          seededCrypto.apiKey(prefix: prefix, length: length, asHex: asHex),
+          equals(
+            seededCrypto.apiKey(prefix: prefix, length: length, asHex: asHex),
+          ),
+        );
+      }
+    });
   });
 }