Add comprehensive authentication documentation and multi-auth support

[hlgr360]

Summary

This PR adds comprehensive authentication documentation and enhances the MCP server to support multiple authentication methods for connecting to MSSQL databases.

Changes Made

Documentation (README.md)

• Added detailed "Authentication Configuration" section with three supported methods:
  • Azure Active Directory (default) - for Azure SQL Database
  • SQL Server Authentication - for traditional SQL Server instances
  • Windows Authentication (NTLM) - for on-premises domain authentication
• Provided complete configuration examples for each authentication method
• Organized configuration parameters into "Common" and "Authentication-Specific" sections
• Updated all MCP configuration examples to be consistent
• Added environment variable documentation for all authentication methods

Code Enhancement (src/index.ts)

• Enhanced createSqlConfig() function to support multiple authentication methods
• Added configurable authentication via AUTH_METHOD environment variable
• Implemented proper token management and connection reuse for Azure AD
• Added support for SQL Server and Windows authentication methods
• Improved connection handling for different authentication types

Environment Variables Added

• AUTH_METHOD: Specifies authentication method (azure-ad, sql, windows)
• SQL_USERNAME & SQL_PASSWORD: For SQL Server authentication
• USERNAME, PASSWORD & DOMAIN: For Windows authentication

Benefits

• Provides flexibility for different deployment scenarios
• Maintains backward compatibility (defaults to Azure AD)
• Improves security by supporting appropriate auth methods per environment
• Clear documentation for easy setup and configuration

Testing

• Tested with MS SQL Server for SQL Server Auth
• Code structure supports SQL Server and Windows auth (requires appropriate environment for testing)