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ARROW-14640 [R] reading data from S3

This PR adds the ability for `proxy_options` to be passed from R (`S3FileSystem$create()`) to C++ (`fs::S3Options.proxy_options`). This functionality is [available in Python](https://arrow.apache.org/docs/python/generated/pyarrow.fs.S3FileSystem.html#pyarrow.fs.S3FileSystem) but not in R.

Some issues I'm not sure about:

- I can test the failing case where there's an invalid URI passed but I'm not sure how to test the passing case (valid proxy URI specified).
- Should the `HTTP_PROXY` and/or the `HTTPS_PROXY` environment variables be read to set the default value (as suggested by the issue opener)? I can see how this would be useful but I'm too new to this to have an opinion about whether we should or shouldn't implement it at the R level.

Closes #11691 from paleolimbot/r-s3-proxy-options

Authored-by: Dewey Dunnington <dewey@fishandwhistle.net>
Signed-off-by: Jonathan Keane <jkeane@gmail.com>