[R] Binding for n_distinct() with no grouping

[asfimport]

ARROW-13620 added a binding for n_distinct() but it only works for grouped aggregation, not whole-table aggregation. 

This works:

Table$create(starwars) %>%
  group_by(homeworld) %>%
  summarise(n_distinct(species)) %>%
  collect()

but this errors:

Table$create(starwars) %>%
  summarise(n_distinct(species)) %>%
  collect()

#> Error: Key error: No function registered with name: count_distinct

Once we have a non-hash count_distinct aggregate kernel in the C++ library (ARROW-14035) we should bind the options for it in the R package and add a test.

Reporter: Ian Cook / @ianmcook
Assignee: Percy Camilo Triveño Aucahuasi / @aucahuasi

Related issues:

• [R] Binding for n_distinct() (relates to)
• [C++][Compute] Implement non-hash count_distinct aggregate kernel (depends upon)

_{Note: This issue was originally created as ARROW-14036. Please see the migration documentation for further details.}

[asfimport]

Neal Richardson / @nealrichardson:
Is this equivalent to group_by(everything) %>% summarize(n_distinct())?

[asfimport]

Ian Cook / @ianmcook:
@nealrichardson  yes

[asfimport]

Ian Cook / @ianmcook:
#11257 (ARROW-14035) resolves this