Add created/modified fields to unixfs files

[jsonsivar]

Since created and modified date fields are file system level attributes, was hoping they could get added to the UnixFS file metadata fields.

We see that a date time field has been added to the JS version of this https://github.com/ipfs/js-ipfs-unixfs/blob/master/packages/ipfs-unixfs/src/unixfs.proto.js#L23 so wanted to open the discussion on what you thought about both created and modified date fields and adding it to the UnixFS specifications.

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[Stebalien]

This feature is being tracked in ipfs/kubo#6920.