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ARROW-16607: [R] Improve KeyValueMetadata handling

* Pushes KVM handling into ExecPlan so that Run() preserves the R metadata we want.
* Also pushes special handling for a kind of collapsed query from collect() into Build().
* Better encapsulate KVM for the the $metadata and $r_metadata so that as a user/developer, you never have to touch the serialize/deserialize functions, you just have a list to work with. This is a slight API change, most noticeable if you were to `print(tab$metadata)`; better is to `print(str(tab$metdata))`.
* Factor out a common utility in r/src for taking cpp11::strings (named character vector) and producing arrow::KeyValueMetadata

The upshot of all of this is that we can push the ExecPlan evaluation into `as_record_batch_reader()`, and all that `collect()` does on top is read the RBR into a Table/data.frame. This means that we can plug dplyr queries into anything else that expects a RecordBatchReader, and it will be (to the maximum extent possible, given the limitations of ExecPlan) streaming, not requiring you to `compute()` and materialize things first.

Closes #13210 from nealrichardson/kvm

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>