SPARK-3648: Provide a script for fetching remote PR's for review

This is a simple script I wrote for fetching remote pull requests.
I found this very useful during code reviews and I think other
developers would as well.

One nice thing is this allows you to avoid creating local refs for
all pull requests, which can be annoying since there is high churn
in PR activity.
Some minor style comments:

You can format this so that each command in the pipeline is on a separate line.

```shell
remote_name=$(
  git remote -v \
  | grep "apache/spark\.git" \
  | grep "(fetch)$" \
  | awk '{ print $1; }'
)
```

May I also suggest quoting the search terms and restricting the search for `fetch` to lines ending in the word.

Finally, if there is any chance this filter might return more than one result, we should probably add `-m 1` to the first grep statement.