[11.x] Add setters to cache stores

[stancl]

This makes it so that each cache store that respects the cache.prefix config also has a setter for the prefix. The prefix can be useful to change at runtime to separate cache data further, e.g. by tenants instead of just by application.

[stancl]

I've added one more setter: setDirectory() in FileStore. FileStore doesn't respect the cache.prefix (since it has no reason to, being application-specific and not a shared resource like Redis), therefore cache can be separated within an application by changing the cache directory instead.

I don't need this change as much as the others so if you don't like it I can revert, but it lets me change the cache directory in our tenancy package directly instead of having to hot-swap the underlying Store for a Cache Repository:

// update the path for the cache store in the config and then:
$newStore = $this->app['cache']->resolve($name);
$repository = $this->app['cache']->store($name);
$repository->setStore($newStore);

Now it's just

$this->app['cache']->store($name)->getStore()->setDirectory($path);

There's already an existing setter for the lock directory, this implements it in the same way (also returns $this).

[stancl]

Thanks!