Simplifies `JoinKeys` from R6 to S3 list-like object #524

Job	Run time
Version Bump 🤜🤛	0s
Audit Dependencies 🕵️‍♂️ / oysteR scan 🦪	0s
Coverage 📔 / Coverage 📔	0s
Coverage 📔 / Covtracer 🐄	0s
gitleaks 💧 / gitleaks 💧	0s
gitleaks 💧 / PII Check 💳	0s
License Check 🃏 / Check 🚩	0s
SuperLinter 🦸‍♀️ / SuperLinter 🦸‍♂️	0s
Check URLs 🌐 / Validate Links 🔎	0s
SuperLinter 🦸‍♀️ / Lint R code 🧶	0s
R CMD Check 🧬 / ${{ matrix.config.image }}, version ${{ matrix.config.tag }}	0s
Roxygen 🅾 / Manual pages check 🏁	0s
Spell Check 🆎 / Check spelling 🔠	0s
Style Check 👗 / Check code style 🔠	0s
Version Check 🏁 / Emoji in NEWS.md 📰	0s
Version Check 🏁 / Version check 🏁	0s
Coverage 📔 / Publish coverage report 📰	0s
R CMD Check 🧬 / Publish JUnit HTML report 📰	0s
R CMD Check 🧬 / Upload build tar.gz	0s
	0s

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