Add NewMysqlBackendWithDB constructor to accept existing database connections

[Copilot]

This PR adds support for creating MySQL backend instances with an existing *sql.DB connection, addressing the need for better database connection management and integration with existing applications.

Changes

Added a new constructor NewMysqlBackendWithDB(db *sql.DB, opts ...option) that allows users to provide their own configured database connection instead of having the backend create one internally.

Key Benefits

• Better connection pool control: Users can configure MaxOpenConns, MaxIdleConns, ConnMaxLifetime, etc.
• Shared connections: Database connections can be shared across multiple components
• Advanced configurations: Support for connection parameters not easily expressed through the original DSN-based constructor
• Integration flexibility: Easier integration with existing database management strategies

Usage Examples

Original constructor (unchanged):

backend := mysql.NewMysqlBackend("localhost", 3306, "user", "password", "dbname")

New constructor:

dsn := "user:password@tcp(localhost:3306)/dbname?parseTime=true&multiStatements=true"
db, err := sql.Open("mysql", dsn)
db.SetMaxOpenConns(25)
db.SetMaxIdleConns(10)
db.SetConnMaxLifetime(time.Hour)

backend := mysql.NewMysqlBackendWithDB(db)

Implementation Details

• Refactored common logic into shared newMysqlBackend internal function
• Updated migration logic to handle both DSN-based and existing DB scenarios
• Maintained full backward compatibility with existing NewMysqlBackend
• Added comprehensive test coverage including integration tests
• Updated documentation with usage examples and important notes

Note: When using NewMysqlBackendWithDB, ensure your connection string includes multiStatements=true if you plan to use automatic migrations (enabled by default).

Fixes #412.

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