[R] feather round-trip support for named vectors in list columns

### Describe the bug, including details regarding any error messages, version, and platform.

Are write_feather and read_feather supposed to support named vectors in list columns? Here they drop the names:

```r
library(arrow)
x <- tibble::tibble(list_column = list(c(a = 1, b = 2)))
write_feather(x, "x.feather")
read_feather("x.feather")$list_column
#> [1]>
#> [[1]]
#> [1] 1 2
x$list_column
#> [[1]]
#> a b
#> 1 2
```

### Component(s)

R

Start by running the R reproduction using write_feather and read_feather, then inspect the R implementation behind those entry points. Done means a named vector in a list column retains its names after the Feather round trip, with coverage for the shown example.

Written by the indexing model from the issue text.