Control handling of survfit prediction beyond last timepoint

[anikoszabo]

Add a summary.args = list(extend = TRUE) argument to ard_survival_survfit
When a user passes summary.args = list(extend = FALSE), we return an ARD with the same structure, but the estimates are NA.

Addresses follow-up comment to issue #224

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Pre-review Checklist (if item does not apply, mark is as complete)

• All GitHub Action workflows pass with a ✅
• PR branch has pulled the most recent updates from master branch: usethis::pr_merge_main()
• If a bug was fixed, a unit test was added.
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Reviewer Checklist (if item does not apply, mark is as complete)

• If a bug was fixed, a unit test was added.
• Code coverage is suitable for any new functions/features: devtools::test_coverage()

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