[R] Improve KeyValueMetadata handling

[asfimport]

Followup to ARROW-15271. Among the objectives:

• Push KVM handling in ExecPlan so that Run() preserves the R metadata we want; also remove the duplicate handling of it for Write()
• Better encapsulate KVM for the the $metadata and $r_metadata so that as a user/developer, you never have to touch the serialize/deserialize functions, you just have a list to work with
• Factor out a common utility in r/src for taking cpp11::strings (named character vector) and producing arrow::KeyValueMetadata

Reporter: Neal Richardson / @nealrichardson
Assignee: Neal Richardson / @nealrichardson

PRs and other links:

• GitHub Pull Request #13210

_{Note: This issue was originally created as ARROW-16607. Please see the migration documentation for further details.}

[asfimport]

Neal Richardson / @nealrichardson:
Issue resolved by pull request 13210
#13210