cpp-api: WasmType/Val does not handle endianness conversions from native types on big-endian platforms

The c-api indicates the [values stored in wasmtime_val_raw_t indicate they must be little-endian](https://github.com/bytecodealliance/wasmtime/blob/554dc00453100c4aa9d9b2557c819ff24e9a7599/crates/c-api/include/wasmtime/val.h#L380).

For the cpp-api, passing native cpp types (such as uint32_t, uint64_t, etc) to [wasmtime::WasmType](https://github.com/bytecodealliance/wasmtime/blob/554dc00453100c4aa9d9b2557c819ff24e9a7599/crates/c-api/include/wasmtime/func.hh#L60-L73) does not perform the endianness conversion on big-endian platforms when storing directly into wasmtime_val_raw_t. I believe wasmtime::Val also needs endianness conversions.

Related: https://github.com/proxy-wasm/proxy-wasm-cpp-host/pull/503#discussion_r2934516888