Fix: getrandom dependency issue in WASM build

[kobby-pentangeli]

Motivation

The ic-crypto-getrandom-for-wasm package, which is a workaround for the getrandom dependency issues in WASM environments, doesn't work in all cases. Sometimes, just including the package in Cargo.toml isn't enough.

This PR provides a fix that changes how this package is consumed...
To register this new custom implementation, call the function inside of your canister's init method or wherever canister initialization happens, with conditional compilation, like this:

#[init]
pub fn init(&mut self) {
    #[cfg(target_family = "wasm")]
    ic_crypto_getrandom_for_wasm::register_custom_getrandom();
}

Changes

• Refactored implementation and export of ic-crypto-getrandom-for-wasm::register_custom_getrandom function.
• Updated crate-level doc to reflect usage example.

Closes

• Ticket: https://infinityswap.atlassian.net/browse/EPROD-828

NOTE

• This implementation has only been tested against a couple of canisters locally. Thorough testing against other canisters is highly recommended.