feat: add carbon support for cache expiry

[innocenzi]

This pull request adds nesbot/carbon support for determining the cache TTL.

I'm used to use now()->addHour() for the TTL on Laravel's caching utilities, so it was a bummer when migrating some code to Saloon to see it doesn't support this.

To achieve support for this feature without breaking change, the Cacheable interface has been changed to remove cacheExpiryInSeconds. Instead, the HasCaching trait uses method_exists to find either the deprecated cacheExpiryInSeconds or the new cacheExpiry method.

The new cacheExpiry can either return an int or a CarbonInterface. In either case, the cache expiry will be resolved to seconds internally to minimize changes to the codebase.

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Note that I have other concerns with the DX, as stated in https://github.com/Sammyjo20/saloon-cache-plugin/issues/11, so maybe it's worth not accepting this PR and make a bigger refactor for a better Laravel integration