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ARROW-17439: [R] Change behavior of pull to compute instead of collect (#14330)

I could use some help with a couple of things:

- [x] `devtools::check` warns about `pull.ArrowTabluar` being undocumented. I `@export`ed it to stay consistent with other ArrowTabular generics defined in `arrow-tabular.R` and don't understand why checking doesn't warn on all of these. Does this one just not need exporting?
    <details>
    <summary>Relevant devtools::check() output</summary>

    ```
    ❯ checking for missing documentation entries ... WARNING
      Undocumented code objects:
        ‘pull.ArrowTabular’
      All user-level objects in a package should have documentation entries.
      See chapter ‘Writing R documentation files’ in the ‘Writing R
      Extensions’ manual.
    ```
    </details>
- [x] I found an inconsistency with `dplyr::pull` and Tables: Pulling an ungrouped Table produces a ChunkedArray whereas pulling a grouped Table produces a Table. This makes a subsequent call to `as.vector` produce an error of `Error in as.vector(x, mode) : cannot coerce type 'environment' to vector of type 'any'`
     <details>
    <summary>Example of the difference</summary>

    ```r
    > sw_table <- arrow_table(starwars)

    > sw_table |>
    +   filter(height > 100) |>
    +   group_by(homeworld) |>
    +   pull(name) |> 
    +   class()
    [1] "Table"        "ArrowTabular" "ArrowObject"  "R6"          

    > sw_table |>
    +   filter(height > 100) |>
    +   pull(name) |> 
    +   class()
    [1] "ChunkedArray" "ArrowDatum"   "ArrowObject"  "R6"
    ```
    </details>


Authored-by: Bryce Mecum <petridish@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>