Cache generation issues

[vvasiloi]

Hi,

While integrating the plugin together with the providers added in #63, we encountered some issues with the caching mechanism:

1. Enabling the cache completely breaks the provider because in our checkout flow there's no shipping address at that point.
   The customers first enter the billing address, then they either add a shipping address or select a pickup point based on the billing address. Ref:

   SyliusPickupPointPlugin/src/Provider/CachedProvider.php

   Lines 96 to 104 in eeacac9

   	private function buildOrderCacheKey(OrderInterface $order): string
   	{
   	$shippingAddress = $order->getShippingAddress();
   	if (!$shippingAddress instanceof AddressInterface) {
   	throw new RuntimeException(sprintf(
   	'Shipping address was not found for order #%s',
   	$order->getNumber()
   	));
   	}

2. It completely breaks because the exceptions thrown during the cache key generation are not caught
3. Because the cache key generation is in private methods, changing that logic would mean copy-pasting the entire class and replacing the service.
4. The CacheProvider decorators have a relatively high priority of 256, which created a bit of confusion because we already decorated the providers with the default priority of 0.

To overcome all these issues, I suggest the following changes:

1. Extract the cache key generation logic in separate classes, so it's easy to change per project or even per provider.
2. Catch errors from cache key generation, log them and bypass the cache.
3. Register the cache decorators with a negative priority.
4. Fallback to the billing address when there's no shipping address on cache key generation.

WDYT?