Allow developer to set the memory id for the ic-wasi-polyfill memory

[lastmjs]

Right now the ic-wasi-polyfill memory that we pass into init_with_memory is hard-coded to 254. This is a sad caveat that we have to explain to the developer. We might want to keep this as the default, but allow the developer to change it, probably in dfx.json.canisters.[canisterName].custom.