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ARROW-6439: [R] Implement S3 file-system interface in R

This patch does the foundational work to conditionally build the S3 bindings if the C++ library was built with ARROW_S3=ON. It also adds bindings for `FileSystemFromUri` and enough wiring up in the datasets code so that `open_dataset("s3://ursa-labs-taxi-data", partitioning = c("year", "month"))` works.

There's lots of other S3FileSystem methods that probably should be implemented and aren't here. Also, calling `S3FileSystem$create()` segfaults. But `FileSystemFromUri` works fine.

Closes #6901 from nealrichardson/r-s3

Lead-authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Co-authored-by: François Saint-Jacques <fsaintjacques@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>