Add Built-in Health Check Support

[v-kamerdinerov]

Adds native health check functionality to the SOCKS5 proxy server, enabling seamless integration with Docker health checks and monitoring systems.

Changes

Added

• --healthcheck flag - Built-in health check command that can be invoked directly from the binary
• Full SOCKS5 protocol handshake - Performs proper SOCKS5 greeting and authentication
• Automatic authentication detection - Supports both authenticated and non-authenticated modes
• Docker Compose integration - Ready-to-use health check configuration

Technical Implementation

• Health check connects to the SOCKS5 port and performs protocol handshake
• Offers both no-auth (0x00) and username/password (0x02) authentication methods
• Automatically authenticates using PROXY_USER and PROXY_PASSWORD environment variables when required
• Returns exit code 0 (success) or 1 (failure) for easy integration with monitoring tools

Benefits

• ✅ No additional HTTP endpoint or port required
• ✅ No spurious errors in server logs during health checks
• ✅ Works with both REQUIRE_AUTH=true and REQUIRE_AUTH=false
• ✅ Simple Docker health check: test: ["/app/socks5", "--healthcheck"]
• ✅ Native Go implementation with no external dependencies

Usage

Standalone:

./socks5 --healthcheck

Docker Compose:

healthcheck:
  test: ["/app/socks5", "--healthcheck"]
  interval: 30s
  timeout: 3s
  retries: 3
  start_period: 10s

Additional Changes

• Code formatting with gofmt
• Add docs to README.md(also added forgot info about PROXY_LISTEN_IP)
• Added CHANGELOG.md entry

[v-kamerdinerov]

@serjs please check this PR