Core abstractions, cluster fault tolerance, filter chain, load balancer, and SPI for small-rpc 2.0

In-memory local service registry implementation for Small RPC

Netty-based network transport and heartbeat module for Small RPC

Spring Framework integration for Small RPC annotations (@RpcService, @RpcReference)

Redis-based service registry implementation with TTL heartbeat for Small RPC

ZooKeeper-based service registry implementation using Apache Curator for Small RPC