Support multiple sql dialects

[runleveldev]

This pull request migrates the authentication and directory backend from MySQL-specific implementations to a unified, database-agnostic SQL backend. It introduces new configuration options for SQL backends, updates documentation and example configs to reflect these changes, and removes the old MySQL backend code in favor of new SQL backend providers. The changes make it easier to support multiple SQL databases (MySQL, MariaDB, SQLite3, PostgreSQL) and simplify configuration via connection URLs and custom SQL queries.

Backend migration and code removal:

• Removed the legacy MySQL authentication and directory provider implementations (server/backends/mysql.auth.js and server/backends/mysql.directory.js). [1] [2]
• Added new SQL authentication provider (server/backends/sql.auth.js) that uses Sequelize and supports custom queries via environment variables.

Configuration and environment variable changes:

• Changed environment variable names and syntax throughout documentation and example files to use sql instead of mysql, and replaced individual MySQL variables with a single SQL_URL and query variables (SQL_QUERY_ONE_USER, etc.). Updated example configs in README.md, .env.example, and other docs. [1] [2] [3] [4]

Documentation updates:

• Updated documentation, migration guides, and examples to reference the new SQL backend and configuration syntax, including supported databases and example queries. [1] [2] [3] [4] [5]

Feature and compatibility improvements:

• The new SQL backend supports MySQL, MariaDB, SQLite3, and PostgreSQL, making the gateway more flexible and easier to integrate with different systems. [1] [2]

Migration instructions:

• Provided clear migration steps and examples for updating from MySQL to SQL backend, including changes to environment variables and query customization. [1] [2] [3]