feat(config): integrate etcd-based configuration management

- Add etcd-client dependency with TLS support
- Implement EtcdConfig struct for reading config values with priority: etcd > env > default
- Add ServiceRegistry for service discovery registration in etcd
- Create from_etcd method in AppConfig for loading SMTP configuration
- Update main.rs to use etcd-based config loading with fallback mechanism
- Add etcd module with client connection and key-value operations
- Modify Dockerfile to use cargo-chef for faster builds
- Add docker-compose.yaml for emailks service deployment
- Include AGENTS.md with development guidelines and best practices
- Add build.sh script for podman-based container building
- Update dependencies in Cargo.toml and Cargo.lock

docker-compose.yaml

@@ -0,0 +1,14 @@
+x-emailks: &emailks
+  image: emailks
+  restart: unless-stopped
+  env_file: .env
+  environment:
+    RUST_LOG: info
+    APP_SMTP_LISTEN_ADDR: 0.0.0.0:50050
+    APP_SMTP_QUEUE_CAPACITY: "1000"
+
+services:
+  emailks:
+    <<: *emailks
+    ports:
+      - "50050:50050"