overlays: file-add should allow replacing existing files (or add a file-replace)

[reubeno]

A common use case for file-add will be to replace an existing file with one of our own containing different content, so it doesn't make sense to require users to do file-remove then immediately file-add with the same file. if we want to leave file-add failing if the file exists, then there should be a file-replace command added that works like file-add but allows replacing an existing file.

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Imported on behalf of @ddstreet