Make SPI_prepare argtypes argument const

Make SPI_prepare argtypes argument const

This changes the argtypes argument of SPI_prepare(),
SPI_prepare_cursor(), SPI_cursor_open_with_args(), and
SPI_execute_with_args() from Oid *argtypes to const Oid *argtypes.
The underlying functions were already receptive to that, so this
doesn't require any significant changes beyond the function signatures
and some internal variables.

Commit 28972b6fc3dc recently introduced a case where a const had to be
cast away before calling these functions.  This is fixed here.

In passing, make a very similar const addition to SPI_modifytuple().

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Ewan Young <kdbase.hack@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/86b5162f-c472-40fa-997b-0450dece1dec%40eisentraut.org