cql3/functions/user_function: Use fmt to format create statement

We replace `std::ostringstream` with views and formatting
using fmt to improve readability of the code.

(cherry picked from commit 1f1b201fd8)

# Conflicts:
#	cql3/functions/user_function.cc

cql3/functions/user_function.cc

@@ -13,6 +13,8 @@
 
 #include <seastar/core/thread.hh>
 
+#include <ranges>
+
 namespace cql3 {
 namespace functions {
 
@@ -79,6 +81,7 @@ std::ostream& user_function::describe(std::ostream& os) const {
     }
     os << ")\n";
 
+<<<<<<< HEAD
     if (_called_on_null_input) {
         os << "CALLED";
     } else {
@@ -92,6 +95,33 @@ std::ostream& user_function::describe(std::ostream& os) const {
        << "$$;";
 
     return os;
+=======
+        auto arg_type_range = _arg_types | std::views::transform(std::mem_fn(&abstract_type::cql3_type_name));
+        auto arg_range = std::views::zip(_arg_names, arg_type_range)
+                | std::views::transform([] (std::tuple<std::string_view, std::string_view> arg) {
+                    const auto [name, type] = arg;
+                    return seastar::format("{} {}", name, type);
+                });
+
+        return seastar::format("CREATE FUNCTION {}.{}({})\n"
+                "{} ON NULL INPUT\n"
+                "RETURNS {}\n"
+                "LANGUAGE {}\n"
+                "AS $${}$$;",
+                cql3::util::maybe_quote(name().keyspace), cql3::util::maybe_quote(name().name), fmt::join(arg_range, ", "),
+                _called_on_null_input ? "CALLED" : "RETURNS NULL",
+                _return_type->cql3_type_name(),
+                _language,
+                _body);
+    });
+
+    return description {
+        .keyspace = name().keyspace,
+        .type = "function",
+        .name = name().name,
+        .create_statement = std::move(maybe_create_statement)
+    };
+>>>>>>> 1f1b201fd8 (cql3/functions/user_function: Use fmt to format create statement)
 }
 
 }