Add Reactive Dapr apps with Drasi sample

# Add Reactive Dapr apps with Drasi sample ## Summary This PR adds a comprehensive e-commerce sample demonstrating how [Drasi](https://drasi.io) (a CNCF project) enhances Dapr applications with real-time, reactive data processing capabilities across multiple microservices. ## Motivation Dapr users often face challenges with: - Creating derived data views across multiple services - Building real-time dashboards without polling - Converting low-level database changes into business events This sample, originally presented at [Dapr Community Call #123](https://www.youtube.com/watch?v=S-ImhYfLplM&t=90s), demonstrates how Drasi solves these challenges while leveraging Dapr's existing capabilities. ## What This Sample Demonstrates ### Scenario An e-commerce platform with 4 existing Dapr microservices (Products, Customers, Orders, Reviews), each with its own PostgreSQL state store. In this example, Drasi allows creation of 3 new services, each demonstrating the new capabilities Drasi provides without modifying the existing services: 1. **Catalog Service** - Materialized view combining products and review statistics 2. **Dashboard Service** - Real-time monitoring of at-risk orders and inventory 3. **Notifications Service** - Intelligent business events via Dapr pub/sub ### Key Features - **Zero-impact monitoring** using PostgreSQL logical replication - **Declarative queries** using Drasi's graph-based query language - **Three Drasi reactions**: Sync Dapr StateStore, SignalR, Post Dapr PubSub - **Complete automation** with k3d cluster and comprehensive Makefile ## Testing - [x] All services deploy and run successfully on k3d - [x] Demo scripts execute without errors - [x] Real-time updates verified in browser - [x] Works with Dapr v1.15.0 - [x] README includes all required elements (version table, instructions, diagrams) ## Checklist - [x] Sample is complete and functional - [x] README with clear instructions - [x] Architecture diagram included - [x] Entry added to main README.md table - [x] All commits signed with DCO ## Related - **Presentation**: [Dapr Community Call #123](https://www.youtube.com/watch?v=S-ImhYfLplM&t=90s) - **Slides**: [Google Slides](https://docs.google.com/presentation/d/1w8Pcu4SKxbu5ygfBo1tLju3YzxznZlw7litKqAS23xI/) - **Drasi Docs**: https://drasi.io Signed-off-by: Aman Singh <your-email@microsoft.com>