Add way to inject caching machanism externally as a interface

[sacOO7]

• In laravel, a new instance is created for every request, so providing an external caching mechanism for remembering and reusing stuff again will be useful. Example - Host Caching (HostCache.php) is used for remembering the last used host/fallback host. So, an external caching mechanism via interface can be provided in laravel.

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The link to the corresponding Jira issue is https://ably.atlassian.net/browse/SDK-3162