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ARROW-8900: [C++][Python] Expose Proxy Options as parameters for S3FileSystem

As discussed in the comments on the JIRA issue, I've added a simple struct called `S3ProxyOptions` which holds the fields for proxy-related information to pass to the AWS SDK when building the S3 client.
I added a simple `FromUri` helper function since these options can be derived from environment variables like `http_proxy`, `HTTP_PROXY`, `HTTPS_PROXY`, etc. I didn't automatically honor these environment variables, but that is something we could do in a follow-up PR (or leave it for the users to do).
Also added the options to the pyarrow interface for the file-system.

I tested the proxy functionality by setting up a simple proxy on my local machine and checking the logs to ensure that the traffic from the file-system was flowing through it.

Closes #9996 from sahil1105/sahil/s3-http-proxy

Lead-authored-by: sjgupta2 <sjgupta2@illinois.edu>
Co-authored-by: Sahil Gupta <sahil1105@hotmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>