Bytes [start, end), clamped to the source size. A caller that needs
every byte it asked for must check the returned length.
Optionalsignal: AbortSignalStaticopenIssue the first range request, learn the file size, keep the prefix.
Optionaloptions: HttpRangeSourceOptions
Random-access byte source: the seam that lets one reader work over a local
File/Blob(BlobByteSource) or a remote file through HTTP Range requests (HttpRangeSource) without ever holding the whole file (#6869).