config not serialisable when doing config:cache

[stephenkhoo]

When we try to optimise the Laravel performance, one of the common practice we do is cache the config.
However, it can't serialise the config as there's a function in the config
Maybe we will need to adjust a little bit, so that we can exclude the need of the callback in the config

Let's say we have another
app/settings/mutator.php

Then in config, we're just pointing towards different key of the array inside the settings/mutator.php