Add GaussDB Database Support

[moseszane168]

• Do only one thing
• Non breaking API changes
• Tested

Add GaussDB Database Support

This PR adds support for Huawei GaussDB as a database dialect in GORM.

What did this pull request do?

• Implemented a new dialect for Huawei GaussDB based on the PostgreSQL dialect
• Added proper type mappings for GaussDB specific data types
• Implemented GaussDB specific SQL differences handling
• Added test cases for GaussDB compatibility
• Updated documentation to include GaussDB support

User Case Description

Many enterprises in China are using Huawei GaussDB as their primary database solution. This PR enables GORM users to seamlessly work with GaussDB while maintaining compatibility with existing PostgreSQL-oriented code.

// Example usage:
import (
  "gorm.io/driver/gaussdb"
  "gorm.io/gorm"
)

func main() {
  dsn := "host=localhost user=gorm password=gorm dbname=gorm port=8000 sslmode=disable"
  db, err := gorm.Open(gaussdb.Open(dsn), &gorm.Config{})
  // ...
}