Break http crate dependency on wasmtime

[itowlson]

I think this might be the last one folks... again I resorted to a feature but I can look at splitting out crates if people think that's better.

[lann]

This is fine but I want to obnoxiously note that if there isn't something checking for this dependency it will likely creep back in.

[itowlson]

@lann Yeah that's my concern. I am not sure how to mitigate it - I could put a silly check into cloud-plugin but that would be a severely lagging indicator, and Spin will never see it because it compiles app/core in anyway. I guess I could write a thingy that references the dependency-broken crates and rings a bell if it ever finds itself importing wasmtime, and include that in Spin CI. It seems awfully specific though.