c/driver/postgresql: Handle non-COPY path results where individual string data would be larger than 2GB

[paleolimbot]

What feature or improvement would you like to see?

In the COPY reader we go to quite some lengths to ensure that we don't overflow the 2GB limit for the Arrow string type:

arrow-adbc/c/driver/postgresql/statement.cc

Lines 770 to 776 in 45cd9be

	na_res = AppendRowAndFetchNext(&error);
	if (na_res == EOVERFLOW) {
	// The result would be too big to return if we appended the row. When EOVERFLOW is
	// returned, the copy reader leaves the output in a valid state. The data is left in
	// pg_buf_/data_ and will attempt to be appended on the next call to GetNext()
	return BuildOutput(out, &error);
	}

This was not implemented in the PR that enabled converting PQresult objects to arrays (#2029). While it is less likely that one of these queries will overflow (given that it is opt-in), we have the infrastructure to handle it and should!