Update RawOutput API (#8075)
### Description
Update `RawOutput` API
Updated API suggested by @sokra in review on #65223:
https://github.com/vercel/next.js/pull/65223#discussion_r1587157258
Without this change, I don't think this API is useful. This makes the API very similar to `VirtualOutputAsset`.
### Testing Instructions
Tested with https://github.com/vercel/next.js/pull/65300
Closes PACK-3034
