Make `create_onion_message` a freestanding function

Make `create_onion_message` a freestanding function

The new `create_onion_message` function in `OnionMessenger` is hard
to handle - it has various generic requirements indirectly via the
struct, but they're not bounded by any of the method parameters.
Thus, you can't simply call `OnionMessenger::create_onion_message`,
as various bounds are not specified.

Instead, we move it to a freestanding function so that it can be
called directly without explicitly setting bounds.