[1.0] Replace `wasi:http@0.3` `handler` / `client` split with single interface using `implements`

[lann]

This split was a workaround for the absence of implements which could be resolved in 1.0:

https://github.com/WebAssembly/WASI/blob/main/proposals/http/wit/worlds.wit#L104-L108

/// The type signature of `client.send` is the same as `handler.handle`. This
/// duplication is currently necessary because some Component Model tooling
/// (including WIT itself) is unable to represent a component importing two
/// instances of the same interface. A `client.send` import may be linked
/// directly to a `handler.handle` export to bypass the network.