[R] Make better use of field metadata

### Describe the enhancement requested

In #49631 we enabled field metadata, i.e.

``` r
library(arrow)
mt_table <- arrow_table(mtcars)
mt_schema <- schema(mt_table)
mt_schema[["mpg"]] <- field("mpg", int32())$WithMetadata(list(description = "miles per gallon"))
mt_table <- arrow_table(mtcars, schema = mt_schema)
mt_table$schema[["mpg"]]$metadata
#> $description
#> [1] "miles per gallon"
```

We should do something more with it so that users can see this information and make better use of it. Before opening a PR we need to work out what the UX should be.

### Component(s)

R

Start with the field metadata flow shown in the issue—arrow_table(), schema(), field(), and the metadata accessor—and review the earlier enablement in #49631. Define the intended R user experience for viewing and using metadata before proposing implementation; the issue names no files or tests, so completion is an agreed UX and follow-up scope.

Written by the indexing model from the issue text.