overlays: file-add source field should be optional

The common use case for `file-add` is probably going to be where `file` == `source`, so it would be good for the `source` field to be optional so we don't always have to specify both of them with the same value

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_Imported on behalf of @ddstreet_

Locate the overlays implementation that handles the `file-add` operation and its `file` and `source` fields. Check how omitted fields are currently validated or interpreted, then verify that omitting `source` uses `file` as the source without changing explicit `source` values.

Written by the indexing model from the issue text.