Buffer file reads in `serde_json::from_reader` (#10341)
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## Summary
https://docs.rs/serde_json/latest/serde_json/fn.from_reader.html
suggests that
> When reading from a source against which short reads are not
efficient, such as a
[File](https://doc.rust-lang.org/std/fs/struct.File.html), you will want
to apply your own buffering because serde_json will not buffer the
input. See
[std::io::BufReader](https://doc.rust-lang.org/std/io/struct.BufReader.html).
Without this buffering, we observe a sequence of single byte reads which
can be quite inefficient depending on the underlying filesystem.
This adds buffering with `std::io::BufReader` to resolve this.
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## Test Plan
Unit tests cover this code.
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