IndexAttribute 优化，提升代码实现可读性

#### Feature 特性

```c#
namespace FreeSql.DataAnnotations;

[AttributeUsage(AttributeTargets.Class, AllowMultiple = true)]
public class IndexAttribute : Attribute
{
public string Fields { get; set; }
}

```

#### 简要描述原因
`Fields` 能否由string改为string[]
```c#
public string[] Fields { get; set; }

public IndexAttribute(string name, params string[] fields)
{
this.Name = name;
this.Fields = fields;
}
```

#### 使用场景

```c#
[Index("{tablename}_user_phone", "user_id", "phone`)]
```