refactor: extract Attr's marshal and unmarshal from baseMeta to Attr

[shenxingwuying]

I think the Attr's marshal and unmarshal in Attr is better than baseMeta. Maybe some other basic structure need review later.

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[jiefenghuang]

Hi @shenxingwuying
Do you need to reference this function? Currently, there’s no need to expose this function.

[shenxingwuying]

Hi @shenxingwuying Do you need to reference this function? Currently, there’s no need to expose this function.

I‘m planning to add a new meta service plugin to make some operations can push to it,
and it's a better way to keep the storage format in juicefs.