Support client-generated IDs when creating resources

[tobyzerner]

As per https://jsonapi.org/format/#crud-creating-client-ids

[tobyzerner]

Likely API:

class FooResource
{
    public function id()
    {
        return Id::make()
            ->get(...)
            ->allowClientGenerated()
            ->default(...)
            ->set(...)
            ->validate(...)
    }
}