apps/httpd: free allocated http_server_control

[tchaikov]

in main(), we create an instance of http_server_control using new, but we never destroy it. this is identified by ASan

==2190125==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 8 byte(s) in 1 object(s) allocated from:
    #0 0x55e21cf487bd in operator new(unsigned long) /home/kefu/dev/llvm-project/compiler-rt/lib/asan/asan_new_delete.cpp:86:3
    #1 0x55e21cf6cf31 in main::$_0::operator()() const::'lambda'()::operator()() const /home/kefu/dev/seastar/apps/httpd/main.cc:121:27
    #2 0x55e21cf6b4cc in int std::__invoke_impl<int, main::$_0::operator()() const::'lambda'()>(std::__invoke_other, main::$_0::operator()() const::'lambda'()&&) /usr/lib/gcc/x86_64-redhat-linux/14/../../../../incl
ude/c++/14/bits/invoke.h:61:14
    #3 0x55e21cf6b46c in std::__invoke_result<main::$_0::operator()() const::'lambda'()>::type std::__invoke<main::$_0::operator()() const::'lambda'()>(main::$_0::operator()() const::'lambda'()&&) /usr/lib/gcc/x86_
64-redhat-linux/14/../../../../include/c++/14/bits/invoke.h:96:14
    #4 0x55e21cf6b410 in decltype(auto) std::__apply_impl<main::$_0::operator()() const::'lambda'(), std::tuple<>>(main::$_0::operator()() const::'lambda'()&&, std::tuple<>&&, std::integer_sequence<unsigned long, .
..>) /usr/lib/gcc/x86_64-redhat-linux/14/../../../../include/c++/14/tuple:2921:14
    #5 0x55e21cf6b3b2 in decltype(auto) std::apply<main::$_0::operator()() const::'lambda'(), std::tuple<>>(main::$_0::operator()() const::'lambda'()&&, std::tuple<>&&) /usr/lib/gcc/x86_64-redhat-linux/14/../../../
../include/c++/14/tuple:2936:14
    #6 0x55e21cf6b283 in seastar::future<int> seastar::futurize<int>::apply<main::$_0::operator()() const::'lambda'()>(main::$_0::operator()() const::'lambda'()&&, std::tuple<>&&) /home/kefu/dev/seastar/include/sea
star/core/future.hh:2005:28
    #7 0x55e21cf6b043 in seastar::futurize<std::invoke_result<main::$_0::operator()() const::'lambda'()>::type>::type seastar::async<main::$_0::operator()() const::'lambda'()>(seastar::thread_attributes, main::$_0:
:operator()() const::'lambda'()&&)::'lambda'()::operator()() const /home/kefu/dev/seastar/include/seastar/core/thread.hh:260:13
    #8 0x55e21cf6ae74 in seastar::noncopyable_function<void ()>::direct_vtable_for<seastar::futurize<std::invoke_result<main::$_0::operator()() const::'lambda'()>::type>::type seastar::async<main::$_0::operator()()
 const::'lambda'()>(seastar::thread_attributes, main::$_0::operator()() const::'lambda'()&&)::'lambda'()>::call(seastar::noncopyable_function<void ()> const*) /home/kefu/dev/seastar/include/seastar/util/noncopyable
_function.hh:129:20
    #9 0x7f5d757a0fb3 in seastar::noncopyable_function<void ()>::operator()() const /home/kefu/dev/seastar/include/seastar/util/noncopyable_function.hh:215:16
    #10 0x7f5d75ef5611 in seastar::thread_context::main() /home/kefu/dev/seastar/src/core/thread.cc:311:9
    #11 0x7f5d75ef50eb in seastar::thread_context::s_main(int, int) /home/kefu/dev/seastar/src/core/thread.cc:287:43
    #12 0x7f5d72f8a18f  (/lib64/libc.so.6+0x5a18f) (BuildId: b098f1c75a76548bb230d8f551eae07a2aeccf06)

so, in this change, let's hold it using a smart pointer, so we can destroy it when it leaves the lexical scope.

[amnonh]

LGTM, this is a demo app, no risk in merging it