[r] Map raw to arrow::binary()

[mojaveazure]

R currently maps raw vectors to arrow::uint8() instead of arrow::binary(); this prevents the data from being read back as a raw or Python bytes for decoding and recovering

> df <- data.frame(soma_joinid = bit64::seq.integer64(0L, 99L), bytes = raw(100L))
> tbl <- arrow::as_arrow_table(df)
> sdf <- SOMADataFrameCreate("df-raw", schema = tbl$schema, domain = list(soma_joinid = c(0L, 100L)))
> sdf$write(tbl)
> sdf$close()
> sdf <- SOMADataFrameOpen(uri)
> sdf$read()$concat()
[1] Table
100 rows x 3 columns
$soma_joinid <int64 not null>
$bytes <uint8>