[R] Make better use of field metadata

[thisisnic]

Describe the enhancement requested

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

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