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ARROW-16887: [R][Docs] Update Filesystem Vignette for GCS (#13601)

This PR:
 * Replaces all references to the Ursa Labs bucket with the new `voltrondata-labs-datasets` bucket.
 * Adds a new `gs_bucket()` function to R, which parallels the design of the `s3_bucket()` function.
 * Updates the `fs.Rmd` vignette to also discuss GCS. Left discussion about authentication for a follow-up, partly because I am still confused how GCP auth works for non-application use.

Authored-by: Will Jones <willjones127@gmail.com>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>