[FEATURE] SOCKS5 Proxy support for SMTP Validation

[emilianocalzada]

[FEATURE] Add SOCKS5 Proxy Support for SMTP Email Validation

New feature request checklist

• I have updated truemail-go to the latest version
• I have read the Contribution Guidelines
• I have read the documentation
• I have searched for existing GitHub issues

Problem Statement

Current Limitation:

• Many internet service providers block port 25, making local SMTP validation impossible
• Using a single IP address for multiple email validations can lead to being blocked by mail servers

Proposed Solution

Add SOCKS5 proxy support to the ConfigurationAttr structure with two possible implementation options:

1. Structured Format:

SmtpProxy struct {
    Host     string
    Port     int
    User     string
    Password string
}

2. URL String Format.
   SmtpProxy: "socks5://user:pass@host:port"

Business Value

• Local Development: Enables developers to test SMTP validation locally even when port 25 is blocked

• Production Scalability: Allows rotating between different IP addresses through proxies to prevent rate limiting and blocks

• Reliability: Improves success rate of email validation at scale

The feature should be optional, maintaining backward compatibility with existing configurations.