[Feature Request] 提交固定值，放一个固定值在schema中，每次提交都带上

[Qquanwei]

• I have searched the issues of this repository and believe that this is not a duplicate.

What problem does this feature solve?

解决提交固定值的场景

What does the proposed API look like?

{
"type": "string",
"value": "fixed value",
"x-hidden": true,
"x-decorator": "FormItem",
"x-component": "Preview.Text"
}

I believe you can use the x-data attribute on the root level of your schema for adding fixed form values. If you want to have a visible preview of the value, I would go with an input which has its disabled prop set.