[4.x] Allow a class to be specified to generate extra static:warm routes

[ryanmitchell]

When using static:warm the command already cleverly works out any urls to hit from the repositories and basic Route::statamic routes.

However if you have a Route::statamic that has dynamic elements, eg Route::statamic('/artist/{artistSlug}/{portfolioType}') then it doesn't know the URLs to hit.

This PR adds the concept of an extra uri generator to the static:warm command, allow you to specify a class with a handle method that returns a collection of (absolute or relative) urls to hit, e.g.

<?php

namespace App\Caching;

class StaticWarmExtras
{
    public function handle()
    {
        return collect(['/path/to/some/page']);
    }
}

This class can be defined in statamic.static_caching.warm_extra_routes_generator eg

    'warm_extra_routes_generator' => \App\Caching\StaticWarmExtras::Class,