Add VDA 5050 message definitions and ros2_medkit Service Interface plugin

[mfaferek93]

Summary

Add VDA 5050 v2.0.0 message definitions as a ROS 2 msgs package and a SOVD Service Interface gateway plugin that exposes medkit entity/fault data via ROS 2 services.

Motivation

VDA 5050 is the industry standard for fleet management of mobile robots. No complete, vendor-neutral VDA 5050 message definitions exist for ROS 2 Jazzy. Adding these to ros2_medkit:

1. Positions ros2_medkit as authority on VDA 5050 + ROS 2
2. Enables connecting VDA 5050 agents to bridge fleet management (VDA 5050/MQTT) with deep diagnostics (SOVD/medkit)
3. The SOVD Service Interface plugin is reusable by any ROS 2 consumer (BT.CPP, PlotJuggler, RTMaps) - not VDA 5050-specific

Scope

New package: ros2_medkit_vda5050_msgs

• 44 ROS 2 message definitions covering all VDA 5050 v2.0.0 types
• 6 top-level messages (Order, State, InstantActions, Connection, Visualization, Factsheet)
• 38 sub-messages (Node, Edge, Action, ActionState, BatteryState, SafetyState, AgvPosition, etc.)
• Generated from official VDA 5050 JSON schemas (MIT, github.com/VDA5050/VDA5050)

New services in ros2_medkit_msgs

• ListEntities.srv - query entity tree via ROS 2 services
• GetEntityData.srv - retrieve entity data
• GetCapabilities.srv - query SOVD capabilities
• EntityInfo.msg - entity descriptor for service responses
• Reuses existing ListFaultsForEntity.srv for fault queries

New plugin: ros2_medkit_sovd_service_interface

• Gateway plugin (GatewayPlugin + dlopen, PLUGIN_API_VERSION = 4)
• Creates 4 ROS 2 service servers exposing medkit data
• Uses PluginContext API (get_entity_snapshot, list_entity_faults, get_type_capabilities)
• 10 unit tests with FakePluginContext

Architecture

Fleet Manager (MQTT) <-> VDA 5050 Agent (out of scope)
                              |
                         ROS 2 services
                              |
                    SOVD Service Interface Plugin (this PR)
                              |
                         medkit gateway

References

• VDA 5050 spec: https://github.com/VDA5050/VDA5050