pdk-proxy-wasm-stub 1.6.0

Due to a linker error in Windows (https://github.com/rust-lang/rust/issues/86125) that fails when importing a crate that has external symbols, even when they are not being used, we need a proxy-wasm-stub for non wasm32 targets that does not define any extern functions in order to be able to run cargo test (which does not and can not use wasm32 as compilation target)