Fix C23 compiler warning

Commit a67a49648d9 fixed a compiler error under C23. After that, there
is one compiler warning left under C23. It has to do with this in
src/include/nodes/pathnodes.h:

struct IndexOptInfo
{
...
/* AM's cost estimator */
/* Rather than include amapi.h here, we declare amcostestimate like
this */
void (*amcostestimate) () pg_node_attr(read_write_ignore);

This no longer works because in C23, because an empty argument list is
now equivalent to (void), rather than an indeterminate one as before.
And so this results in an incompatible function pointer type and
compiler warnings. (gcc and clang agree on this.)

I think we can fix this easily with a few struct forward declarations,
preserving the goal of not including extra header files, like this:

struct IndexPath;
struct PlannerInfo;

struct IndexOptInfo
{
...
/* AM's cost estimator */
/* Rather than include amapi.h here, we declare amcostestimate like
this */
void (*amcostestimate) (struct PlannerInfo *, struct
IndexPath *, double, Cost *, Cost *, Selectivity *, double *, double *)
pg_node_attr(read_write_ignore);

That way the function pointer has the correct type. This works in older
versions of C as well.