ssx/async_algorithm: disallow passing a coroutine to ssx::async_for_each

In fact fail the compliation if the function returns non-void [[nodiscard]],
which holds for coroutines thanks to ss::future class [[nodiscard]] annotation.

Calling ssx::async_for_each of course does not make much sense, as its purpose
is to inject scheduling points. However it can be done by mistake, e.g. when
a function return type was changed from void to ss::future<>.

src/v/ssx/async_algorithm.h

@@ -106,6 +106,10 @@ template<std::random_access_iterator I, typename Fn>
 iter_size<I> for_each_limit(const I begin, const I end, ssize_t limit, Fn f) {
     auto chunk_size = std::min(limit, end - begin);
     I chunk_end = begin + chunk_size;
+    // std::for_each discards [[nodiscard]] callable result, check explicitly
+    if (false) {
+        f(*begin);
+    }
     std::for_each(begin, chunk_end, std::move(f));
     return {chunk_end, chunk_size};
 }