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ARROW-9859: [C++] Decode username and password in URIs

Allow passing a %-encoded secret key in a S3 URI.
Also, detect and report unrecognized options in S3 URIs.

Closes #8185 from pitrou/ARROW-9859-s3-uri-decode-key

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>