adding `ard_survival_survfit_diff()` function #629

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Audit Dependencies 🕵️‍♂️ / oysteR scan 🦪	1m 39s
Coverage 📔 / Coverage 📔	3m 12s
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Grammar Check 🔤 / Check 📝	12s
gitleaks 💧 / gitleaks 💧	4s
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License Check 🃏 / Check 🚩	34s
Check URLs 🌐 / Validate Links 🔎	6s
Roxygen 🅾 / Manual pages check 🏁	2m 38s
R CMD Check 🧬 / ghcr.io/insightsengineering/rstudio, version latest	3m 46s
Spell Check 🆎 / Check spelling 🔠	1m 39s
Style Check 👗 / Check code style 🔠	1m 42s
Version Check 🏁 / Emoji in NEWS.md 📰	6s
Version Check 🏁 / Version check 🏁	7s
Coverage 📔 / Publish coverage report 📰	0s
R CMD Check 🧬 / Publish JUnit HTML report 📰	0s
R CMD Check 🧬 / Upload build tar.gz	0s
	15m 45s

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