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ARROW-11176: [R] Expose memory pool name and document setting it

Also:

* Exposes `SupportedBackendNames` from the C++ library (perhaps it should be renamed and include "memory" in it, was just following what the internal functions were calling it) so we can make that visible at higher layers
* Adds a general `arrow_info()` function that includes memory pool settings, build-time feature availability (compression codecs, S3), etc., which could be useful for debugging purposes.

Closes #9170 from nealrichardson/r-toggle-pool

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>