Add MySQL driver support to gorm_client

[Copilot]

Adds MySQL as a supported database driver alongside PostgreSQL and SQLite.

Changes

• gorm_client/db.go: Added openMysql() function with standard DSN format (charset=utf8mb4&parseTime=True&loc=Local)
• gorm_client/db.go: Added "mysql" case to NewDB() switch
• go.mod: Added gorm.io/driver/mysql v1.6.0 dependency
• gorm_client/db_test.go: Added unit tests for all database drivers
• gorm_client/example_test.go: Added MySQL usage example
• README.md: Updated supported drivers list

Usage

db := gorm_client.NewDB(gorm_client.Config{
    Driver:   "mysql",
    Host:     "localhost",
    Port:     3306,
    Username: "user",
    Password: "password",
    Name:     "mydb",
})

Original prompt

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This section details on the original issue you should resolve

<issue_title>Support MySQL driver in gorm_client</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Support MySQL driver in gorm_client #15

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