Support VFS-based read and write connections

[eddelbuettel]

This PR adds the ability to read and write single files (in text or binary format) from VFS-supported backends. A simple illustration is

uri <- "s3://tiledb-dirk/tempfolder/penguins.csv"
pp <- read.csv(vfs_file(uri))
summary(pp)

At this point, compression is not supported, nor is nesting connections inside other connections.