Improve mocking of wasi:http/outgoing-handler #88
Description
Currently the wasi:http/outgoing-handler interface implementation relies on fermyon:spin-wasi-virt/http-handler which allows users to supply a response given a url string. This is not nearly as flexible as we want this to be as we want to allow testers to supply different responses across all aspects of an outgoing-request including method, headers, body, etc.