feat(candid): add random value generator

[chenyan-dfinity]

Add a "Lucky" button to generate random primitive values when the field is empty.

[hansl]

Is this really the behaviour we want? I would say fields are mandatory.

Also, empty string is still a valid string.

[chenyan-dfinity]

Fixed.

[hansl]

Suggested change

	const num = Math.floor(Math.random() * 100);
	if (signed && Math.random() < 0.5) {
	return -num;
	} else {
	return num;
	}
	return 100 - Math.floor(Math.random() * 200);

[chenyan-dfinity]

We don't want negative numbers for Nat.