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ARROW-9870: [R] Friendly interface for filesystems (S3)

* Use `SubTreeFileSystem` class to represent a filesystem+path (since that's what it contains already) and wire that up in the file/dataset reader/writer functions that in a recent PR all got a `filesystem` argument added to their signatures (removing the extra argument). Add `FileSystem$path(string)` helper method to create a STFS, and recommend that as the way to pass a filesystem+path to those functions. I recognize that this is an abuse of STFS and will revisit in the future (ARROW-10254), but for now, this yields an improved interface.
* Add `s3_bucket()` as a function to create a STFS containing an `S3FileSystem` and the bucket's path, which also takes advantage of FileSystemFromUri's ability to auto-detect the bucket's region while also allowing you to specify extra S3Options for authentication etc.
* Revise `copy_files()` to take these inputs
* `FileSystem$ls()` as convenience over FileSelector + GetFileInfo etc.
* Update vignettes to reflect these interfaces

Closes #8351 from nealrichardson/r-fs-ui

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