feat(mqtt): Contracts options DTO + enums (name-serialized)

Task 1 of the MQTT/Sparkplug B driver plan: MqttMode, MqttPayloadFormat,
MqttProtocolVersion enums + MqttDriverOptions (broker conn + mode +
nullable Sparkplug/Plain sub-options) per design doc §5.1. Removes the
Task 0 temporary MQTTnet PackageReference from .Contracts (transport-free;
references only Core.Abstractions). MQTTnet stays pinned in
Directory.Packages.props for the .Driver project (Task 3).

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Contracts/MqttProtocolVersion.cs

@@ -0,0 +1,14 @@
+namespace ZB.MOM.WW.OtOpcUa.Driver.Mqtt;
+
+/// <summary>
+///     MQTT wire protocol version negotiated at CONNECT. See
+///     <c>docs/plans/2026-07-15-mqtt-sparkplug-driver-design.md</c> §5.1.
+/// </summary>
+public enum MqttProtocolVersion
+{
+    /// <summary>MQTT 3.1.1.</summary>
+    V311,
+
+    /// <summary>MQTT 5.0.</summary>
+    V500,
+}