FileStream: Buffer more than one `FileOpenFuture`

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Follow up to https://github.com/apache/arrow-datafusion/pull/5161#discussion_r1097183956

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Instead of prefetching just the next stream, buffer up to `N` (where N is configurable?) open files. So maybe break this into two pipelined streams `FileOpenStream` and `FileDecodeStream` then we can used `StreamExt::buffered` to parallelize the opening.

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Do nothing

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Start with the FileStream and FileOpenFuture entry points, then review the linked pull-request discussion for the existing prefetch behavior. Determine how FileOpenStream, FileDecodeStream, and StreamExt::buffered could fit together, and define how configurable N should work; done means multiple file opens can be buffered and parallelized without changing decoding behavior.

Written by the indexing model from the issue text.