rgw: switch back to boost::asio for spawn() and yield_context

rgw: switch back to boost::asio for spawn() and yield_context

a fork of boost::asio::spawn() was introduced in 2020 with spawn::spawn() from #31580. this fork enabled rgw to customize how the coroutine stacks are allocated in order to avoid stack overflows in frontend request coroutines. this customization was based on a StackAllocator concept from the boost::context library

in boost 1.80, that same StackAllocator overload was added to boost::asio::spawn(), along with other improvements like per-op cancellation. now that boost has everything we need, switch back and drop the spawn submodule

this required switching a lot of async functions from async_completion<> to async_initiate<>. similar changes were necessary to enable the c++20 coroutine token boost::asio::use_awaitable

Signed-off-by: Casey Bodley <cbodley@redhat.com>