[Feature Request] Support default value definition of field

[qishipengqsp]

需求描述

• 支持设置属性的默认值，在schema中指定某个属性存在默认值后，可设定默认值
• 默认值可以修改，建议默认值每次都落盘
• 当前属性支持optional，需考虑与optional（nullable）的兼容性问题

注：如有需要，可以在开始这项工作前联系我们

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Brief Description

• Support to define fields having default values
• Default values can be modified. It's suggested that default value is written to storage.
• Compatibility with optional setting of field should be considered.

Note: You can contact the team before you start this if you need.