[1.0] Replace wasi:http@0.3 handler / client split with single interface using implements #948

/// 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.