[R] Allow GcsFileSystem$create to accept a path to json_credentials

[amoeba]

Describe the enhancement requested

As discovered in #33106, GcsFileSystem$create is documented to say it takes a path to a set of JSON credentials on disk. Right now, it only takes a character vector containing the JSON credentials.

So this works:

arrow::gs_bucket("my-gcs_bucket", json_credentials = readr::read_file("my-service_account.json"))

And this does not:

arrow::gs_bucket("my-gcs_bucket", json_credentials = "my-service_account.json")

I think both use cases should work so I think we should:

1. Fix the documentation in [R] Documentation for json_credentials is misleading #33106 so it's clear both approaches work
2. Expand the behavior of GcsFileSystem$create to handle a path to credentials or a string containing the credentials

Component(s)

R

[amoeba]

take