feat: implement mqtt packet-level parser and writer

src/NATS.Server/Mqtt/MqttPacketWriter.cs

@@ -0,0 +1,38 @@
+namespace NATS.Server.Mqtt;
+
+public static class MqttPacketWriter
+{
+    public static byte[] Write(MqttControlPacketType type, ReadOnlySpan<byte> payload, byte flags = 0)
+    {
+        if (type == MqttControlPacketType.Reserved)
+            throw new ArgumentOutOfRangeException(nameof(type), "MQTT control packet type must be non-zero.");
+
+        var remainingLength = payload.Length;
+        var encodedRemainingLength = EncodeRemainingLength(remainingLength);
+        var buffer = new byte[1 + encodedRemainingLength.Length + remainingLength];
+        buffer[0] = (byte)(((byte)type << 4) | (flags & 0x0F));
+        encodedRemainingLength.CopyTo(buffer.AsSpan(1));
+        payload.CopyTo(buffer.AsSpan(1 + encodedRemainingLength.Length));
+        return buffer;
+    }
+
+    internal static byte[] EncodeRemainingLength(int value)
+    {
+        if (value < 0 || value > 268_435_455)
+            throw new ArgumentOutOfRangeException(nameof(value), "MQTT remaining length must be between 0 and 268435455.");
+
+        Span<byte> scratch = stackalloc byte[4];
+        var index = 0;
+
+        do
+        {
+            var digit = (byte)(value % 128);
+            value /= 128;
+            if (value > 0)
+                digit |= 0x80;
+            scratch[index++] = digit;
+        } while (value > 0);
+
+        return scratch[..index].ToArray();
+    }
+}