feat: add Connection to execute multiple commands in a single conn

[ghost]

• [×] Do only one thing
• [×] Non breaking API changes
• [×] Tested

What did this pull request do?

• en : feat: add Connection to execute multiple commands in a single conn
• zh : 支持一个 conn 内可以执行多个命令

User Case Description

func TestWithSingleConnection(t *testing.T) {

	var expectedName = "test"
	var actualName string

	setSQL, getSQL := getSetSQL(DB.Dialector.Name())
	if len(setSQL) == 0 || len(getSQL) == 0 {
		return
	}

	err := DB.Connection(func(tx *gorm.DB) error {
		if err := tx.Exec(setSQL, expectedName).Error; err != nil {
			return err
		}

		if err := tx.Raw(getSQL).Scan(&actualName).Error; err != nil {
			return err
		}
		return nil
	})

	if err != nil {
		t.Errorf(fmt.Sprintf("Connection should work, but got err %v", err))
	}

	if actualName != expectedName {
		t.Errorf("Connection() method should get correct value, expect: %v, got %v", expectedName, actualName)
	}

}