fix: correct iat claim handling and expand allowed claim value types

[kimzuni]

If schema is undefined, claim values should support not only string and number, but also boolean, null, array and object.

This also resolves the TypeScript error for jwt.sign({ iat: boolean }).

before

const jwt = plugin({
	secret: "x",
}).decorator.jwt;

const token = await jwt.sign({
	iat: false, // ts(2345) error
});

after

const jwt = plugin({
	secret: "x",
}).decorator.jwt;

const token = await jwt.sign({
	iat: false, // ok
	// iat: {}, // disallowed
	custom: null, // or string, number, boolean, [], {} // ok
	deep: { key: [{ custom: {} }] }, // ok
});