adding `ard_survival_survfit_diff()` function #615

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gitleaks 💧 / gitleaks 💧	5s
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Check URLs 🌐 / Validate Links 🔎	4s
Roxygen 🅾 / Manual pages check 🏁	2m 37s
R CMD Check 🧬 / ghcr.io/insightsengineering/rstudio, version latest	3m 43s
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Style Check 👗 / Check code style 🔠	1m 59s
Version Check 🏁 / Emoji in NEWS.md 📰	6s
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R CMD Check 🧬 / Publish JUnit HTML report 📰	0s
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	16m 15s

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