plsample: Use TextDatumGetCString() for text-to-CString conversion

plsample: Use TextDatumGetCString() for text-to-CString conversion

Replace the outdated DatumGetCString(DirectFunctionCall1(textout, ...))
pattern with TextDatumGetCString(). The macro is the modern, more
efficient way to convert a text Datum to a C string as it avoids
unnecessary function call machinery and handles detoasting internally.

Since plsample serves as reference code for extension authors, it
should follow current idiomatic practices.

Author: Amul Sul <sulamul@gmail.com>
Discussion: https://postgr.es/m/CAAJ_b95-xMvUN1PEqxv8y6g-A-8k+fSgyv20kSZc9eF1wZAUPg@mail.gmail.com