Improve the OutParam API for wasi-http

[rylev]

This API is a bit awkward but this makes it slightly better by allowing method call syntax. I'm not sure I fully understand why the wit-bindgen generated version does allow for method call syntax.

[dicej]

WIT has three types of functions that can be associated with resources: constructors, methods, and static methods. Methods always take their implicit self/this parameter by borrow handle. Static methods don't have an implicit self/this parameter. There's simply no way (currently) to associate a non-static method with a resource that takes its self/this parameter by own handle, so the closest thing you can do is define a static method that has an explicit self/this parameter as an own handle.

We could add some kind of heurstic to wit-bindgen to detect that pattern and turn it into a method that takes self by owned value, but for now we treat all static methods as associated functions that do not have a real self parameter.