feat(dtls): add dtls client-server handshake flow

Implement initial DTLS handshake flow for server and client using
pion/dtls. Load server and client configuration from .env/environment,
including new debug flags and logging config.

On the server:
- load ServerConfig from env, including DTLS listen addr and debug flag
- create DTLS listener with optional self-signed localhost cert in debug
- accept DTLS sessions and run PerformServerHandshake with a dummy
  domain validator

On the client:
- load ClientConfig from env, then override with CLI flags where given
- validate required fields: server_addr, domain, api_key, local_target
- create DTLS client and run PerformClientHandshake
- support debug mode to skip server certificate verification

Also:
- update go.mod/go.sum with pion/dtls and related dependencies
- extend .env.example with new ports, client config, and debug flags
- ignore built binaries via bin/ in .gitignore

BREAKING CHANGE: client environment variables have changed. The former
HOP_CLIENT_ID, HOP_CLIENT_AUTH_TOKEN and HOP_CLIENT_SERVICE_PORTS are
replaced by HOP_CLIENT_DOMAIN, HOP_CLIENT_API_KEY,
HOP_CLIENT_LOCAL_TARGET and HOP_CLIENT_DEBUG. Client startup now
requires server_addr, domain, api_key and local_target to be provided
(via env or CLI).

.env.example

@@ -30,13 +30,13 @@ HOP_LOKI_ENABLE=false
 # ---- Server ports & domains ----
 
 # HTTP 리스닝 포트 (보통 :80, ACME HTTP-01 및 HTTPS 리다이렉트용)
-HOP_SERVER_HTTP_LISTEN=:80
+HOP_SERVER_HTTP_LISTEN=:8080
 
 # HTTPS 리스닝 포트 (보통 :443)
-HOP_SERVER_HTTPS_LISTEN=:443
+HOP_SERVER_HTTPS_LISTEN=:8443
 
 # DTLS 리스닝 포트 (보통 :443, 필요시 별도 포트 사용)
-HOP_SERVER_DTLS_LISTEN=:443
+HOP_SERVER_DTLS_LISTEN=:8443
 
 # 메인 도메인 (예: example.com)
 HOP_SERVER_DOMAIN=example.com
@@ -45,19 +45,25 @@ HOP_SERVER_DOMAIN=example.com
 # 예: api.example.com,edge.example.com
 HOP_SERVER_PROXY_DOMAINS=api.example.com,edge.example.com
 
+# 디버깅용 플래그
+# 1. self signed localhost cert 사용여부 (debug: true -> 사용)
+HOP_SERVER_DEBUG=true
 
 # ---- Client settings ----
 
 # DTLS 서버 주소 (host:port)
 # 예: example.com:443
-HOP_CLIENT_SERVER_ADDR=example.com:443
+HOP_CLIENT_SERVER_ADDR=localhost:8443
 
-# 클라이언트 식별자
-HOP_CLIENT_ID=client-1
+# 클라이언트 도메인
+HOP_CLIENT_DOMAIN=test.example.com
 
-# 선택적 인증 토큰 (서버에서 검증용으로 사용 가능)
-HOP_CLIENT_AUTH_TOKEN=
+# 인증 토큰 (서버에서 검증용으로 사용 가능)
+HOP_CLIENT_API_KEY=TEST_API_KEY_0123456789_ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789
 
-# 서비스 매핑: name=host:port 형태, 콤마 구분
-# 예: web=127.0.0.1:8080,admin=127.0.0.1:9000
-HOP_CLIENT_SERVICE_PORTS=web=127.0.0.1:8080,admin=127.0.0.1:9000
+# 서비스 매핑: name=host:port 형태
+HOP_CLIENT_LOCAL_TARGET=127.0.0.1:8080
+
+# 디버깅용 플래그
+# 1. self signed 인증서를 신뢰(인증서 체인 검증 스킵)
+HOP_CLIENT_DEBUG=true