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).

.gitignore

@@ -190,3 +190,6 @@ $RECYCLE.BIN/
 *.lnk
 
 # End of https://www.toptal.com/developers/gitignore/api/go,goland+all,dotenv,macos,linux,windows
+
+# builded binary
+bin/