feat(crypto): ✨ add SecretBox related APIs for encrypt and decrypt

Runtime/Algorand.Unity.Crypto/SecretBox/SecretBox+Key.cs

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+using System;
+using System.Runtime.InteropServices;
+
+namespace Algorand.Unity.Crypto
+{
+    public static partial class SecretBox
+    {
+        [StructLayout(LayoutKind.Explicit, Size = SizeBytes)]
+        public unsafe struct Key
+        {
+            public const int SizeBytes = sodium.crypto_secretbox_KEYBYTES;
+
+            [FieldOffset(0)] public fixed byte bytes[SizeBytes];
+
+            public static implicit operator Span<byte>(Key key)
+            {
+                return new Span<byte>(key.bytes, SizeBytes);
+            }
+        }
+    }
+}

Runtime/Algorand.Unity.Crypto/SecretBox/SecretBox+Mac.cs

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+using System;
+using System.Runtime.InteropServices;
+
+namespace Algorand.Unity.Crypto
+{
+    public static partial class SecretBox
+    {
+        [StructLayout(LayoutKind.Explicit, Size = SizeBytes)]
+        public unsafe struct Mac
+        {
+            public const int SizeBytes = sodium.crypto_secretbox_MACBYTES;
+
+            [FieldOffset(0)] public fixed byte bytes[SizeBytes];
+
+            public static implicit operator Span<byte>(Mac mac)
+            {
+                return new Span<byte>(mac.bytes, SizeBytes);
+            }
+        }
+    }
+}

Runtime/Algorand.Unity.Crypto/SecretBox/SecretBox+Nonce.cs

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+using System;
+using System.Runtime.InteropServices;
+
+namespace Algorand.Unity.Crypto
+{
+    public static partial class SecretBox
+    {
+        [StructLayout(LayoutKind.Explicit, Size = SizeBytes)]
+        public unsafe struct Nonce
+        {
+            public const int SizeBytes = sodium.crypto_secretbox_NONCEBYTES;
+
+            [FieldOffset(0)] public fixed byte bytes[SizeBytes];
+
+            public static implicit operator Span<byte>(Nonce nonce)
+            {
+                return new Span<byte>(nonce.bytes, SizeBytes);
+            }
+        }
+    }
+}

Runtime/Algorand.Unity.Crypto/SecretBox/SecretBox.cs

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+using System;
+
+namespace Algorand.Unity.Crypto
+{
+    public static partial class SecretBox
+    {
+        public enum EncryptError
+        {
+            Error = -1,
+            None
+        }
+
+        public enum DecryptError
+        {
+            Error = -1,
+            None
+        }
+
+        public static int CipherLength(int messageLength)
+        {
+            return messageLength + Mac.SizeBytes;
+        }
+
+        public static int MessageLength(int cipherLength)
+        {
+            return cipherLength - Mac.SizeBytes;
+        }
+
+        public static unsafe EncryptError Encrypt(
+            Span<byte> cipher,
+            ReadOnlySpan<byte> message,
+            Key* key,
+            out Nonce nonce
+        )
+        {
+            nonce = Random.Bytes<Nonce>();
+            fixed (byte* noncePtr = nonce.bytes)
+            fixed (byte* cipherPtr = cipher)
+            fixed (byte* messagePtr = message)
+            {
+                return (EncryptError)sodium.crypto_secretbox_easy(
+                    cipherPtr,
+                    messagePtr,
+                    (ulong)message.Length,
+                    noncePtr,
+                    key
+                );
+            }
+        }
+
+        public static unsafe DecryptError Decrypt(
+            Span<byte> message,
+            ReadOnlySpan<byte> cipher,
+            Key* key,
+            Nonce nonce
+        )
+        {
+            fixed (byte* messagePtr = message)
+            fixed (byte* cipherPtr = cipher)
+            {
+                return (DecryptError)sodium.crypto_secretbox_open_easy(
+                    messagePtr,
+                    cipherPtr,
+                    (ulong)cipher.Length,
+                    nonce.bytes,
+                    key
+                );
+            }
+        }
+    }
+}