Upstream duckdb patches for external pointers and `BEGIN_/END_CPP11`

[krlmlr]

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Happy to provide more motivation if and when an implementation can be attempted realistically.

• duckdb/duckdb-r@6f4979d (#387): Support overriding what's passed to R_MakeExternalPtr()
• duckdb/duckdb-r@fe615c4 (#387): Support using BEGIN_CPP11 and END_CPP11 in user code that does not return an SEXP by providing END_CPP11_EX variants that change the return value for the "happy" path. (Why does END_CPP11 need the return anyway?)
• duckdb/duckdb-r@f05d56c (#387): Resolves #308