extract common code of growVector and copyAsPlain into helper

[ben-schwen]

Closes #6908

It dedoubles the code, but due to the recursive call of copyAsPlain, I'm not sure its really cleaner.

Maybe we should add a comment in both functions, linking to each other to not forget updating the code of one, if we update code of the other, and close the issue?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.00%. Comparing base (0216983) to head (771a6fe).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7602      +/-   ##
==========================================
- Coverage   99.00%   99.00%   -0.01%     
==========================================
  Files          87       87              
  Lines       16893    16884       -9     
==========================================
- Hits        16725    16716       -9     
  Misses        168      168              

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[github-actions]

No obvious timing issues in HEAD=unify_growVector_copyAsPlain

Generated via commit 771a6fe

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	2 minutes and 53 seconds
Installing different package versions	22 seconds
Running and plotting the test cases	4 minutes and 8 seconds