[R] Allow setting field metadata

[asfimport]

Currently, can't create a Field with metadata, which makes it hard to create tests regarding field metadata. 

Reporter: Will Jones / @wjones127

_{Note: This issue was originally created as ARROW-18204. Please see the migration documentation for further details.}

[asfimport]

Dewey Dunnington / @paleolimbot:
We should totally support this!

A workaround in case you need it:

library(arrow, warn.conflicts = FALSE)
#> Some features are not enabled in this build of Arrow. Run `arrow_info()` for more information.
# remotes::install_github("paleolimbt/narrow")
library(narrow)

set_field_metadata <- function(field, ...) {
  vals <- rlang::list2(...)
  cschema <- narrow::as_narrow_schema(field)
  current_vals <- cschema$metadata
  keys <- union(names(vals), names(current_vals))
  cschema$metadata <- c(vals, current_vals)[keys]
  arrow::Field$import_from_c(cschema)
}

field_metadata <- function(field) {
  narrow::as_narrow_schema(field)$metadata
}

(f <- field("some name", int32()))
#> Field
#> some name: int32
f_meta <- set_field_metadata(f, some_key = "some value")

field_metadata(f_meta)
#> $some_key
#> [1] "some value"

[thisisnic]

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