[RORDEV-1368] PoC: Fleet support (8.19.7 example)

[coutoPL]

Fleet + APM Demo Stack with ReadonlyREST

This PR contains a complete Elastic Fleet and APM demonstration environment with ReadonlyREST integration.

Stack Components

Core Services:

• Elasticsearch (latest 8.19.7) with ReadonlyREST plugin
• Kibana (latest 8.19.7) with ReadonlyREST plugin
• Fleet Server - Manages Elastic Agents
• Elastic Agent (agent1) - Runs APM Server and system monitoring

Demo Applications:

• service1 - Node.js Express app instrumented with Elastic APM
• traffic-simulator - Generates HTTP traffic to service1 (90% normal requests, 10% errors)

Supporting Services:

• fleet-initializer - Bootstraps Fleet policies and APM integration on startup

What It Demonstrates

✅ Fleet Management - Agent enrollment, policy management, and centralized configuration
✅ APM Monitoring - Real-time application performance monitoring with distributed tracing
✅ ReadonlyREST Security - ACL rules for Fleet, APM, Kibana, and agent authentication
✅ TLS Security - Full certificate-based encryption between all components
✅ System Metrics - CPU, memory, disk, and network monitoring via Elastic Agent

Quick Start

Running the PoC:

1. cd ror-fleet-example
2. ./run.sh
3. Open https://localhost:5601 and log in as admin:admin

Cleaning up:

1. ./clean.sh

Key Features

• Auto-configuration: Fleet policies and APM integration are created automatically
• Version detection: APM package version is auto-detected for compatibility
• Network isolation: All services communicate over a dedicated Docker network
• Persistent storage: Elasticsearch data survives container restarts