Avoid premature release for external pointers by using cpp4r instead of cpp11

[pachadotdev]

Hi

I saw this PR from around 1 yr ago by @krlmlr r-lib/cpp11#423

After talking to Hadley, his standpoint is that I create my own cpp11 fork because I need to move forward with the PhD thesis and adding my PRs to cpp11 would take significant review time.

Being an early DuckDB user, I am happy to contribute back. Perhaps I can get https://github.com/pachadotdev/cpp4r on CRAN and then DuckDB can use it for the R integration?

cpp4r is a drop-in replacement for cpp11, like MariaDB is to MySQL.