GH-38376 [R]: Add dimnames method to Dataset class (#38377)

### Rationale for this change

Add `dimnames` method so that things like `colnames(ds)` work out of the box on datasets. Inspired by dbplyr's lazy tbl implementation

### What changes are included in this PR?

An additional `dimnames` method for Dataset objects

### Are these changes tested?

Yes

### Are there any user-facing changes?

Yes
* Closes: #38376

Authored-by: Jonathan Keane <jkeane@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>

r/NAMESPACE

@@ -101,6 +101,7 @@ S3method(dim,RecordBatchReader)
 S3method(dim,StructArray)
 S3method(dim,arrow_dplyr_query)
 S3method(dimnames,ArrowTabular)
+S3method(dimnames,Dataset)
 S3method(head,ArrowDatum)
 S3method(head,ArrowTabular)
 S3method(head,Dataset)

r/R/dataset.R

@@ -527,6 +527,9 @@ names.Dataset <- function(x) names(x$schema)
 #' @export
 dim.Dataset <- function(x) c(x$num_rows, x$num_cols)
 
+#' @export
+dimnames.Dataset <- function(x) list(NULL, names(x))
+
 #' @export
 c.Dataset <- function(...) Dataset$create(list(...))
 

r/tests/testthat/test-dataset.R

@@ -334,6 +334,12 @@ test_that("dim method returns the correct number of rows and columns", {
   expect_identical(dim(ds), c(20L, 7L))
 })
 
+test_that("dimnames, colnames on Dataset objects", {
+  ds <- open_dataset(dataset_dir, partitioning = schema(part = uint8()))
+  col_names <- c("int", "dbl", "lgl", "chr", "fct", "ts", "part")
+  expect_identical(dimnames(ds), list(NULL, col_names))
+  expect_identical(colnames(ds), col_names)
+})
 
 test_that("dim() correctly determine numbers of rows and columns on arrow_dplyr_query object", {
   ds <- open_dataset(dataset_dir, partitioning = schema(part = uint8()))