[R] Implement Duration R6 class

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Reporter: Nicola Crane / @thisisnic
Assignee: Dewey Dunnington / @paleolimbot

PRs and other links:

• GitHub Pull Request #11850

_{Note: This issue was originally created as ARROW-14941. Please see the migration documentation for further details.}

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Jonathan Keane / @jonkeane:
Issue resolved by pull request 11850
#11850

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Antoine Pitrou / @pitrou:
@jonkeane Please make sure the issue is properly assigned after merging :-)

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Dragoș Moldovan-Grünfeld / @dragosmg:
Correct me if I am wrong, but I don't think this PR implements a binding for {}lubridate::duration(){}. 

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Alenka Frim / @AlenkaF:
@dragosmg see #11850 (comment) and https://issues.apache.org/jira/browse/ARROW-15098 .

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Dragoș Moldovan-Grünfeld / @dragosmg:
Cool. Thanks. I somehow missed that. I've now closed the duplicate issue.