Firm up Callback arguments and document `$context` parameter.

[gsteel]

Changes the Callback validator so that the callable always receives (mixed $value, array $context, ...$otherArgs) - this stabilizes the args for all environments, i.e. standalone usage and when part of input-filter etc.

Because the Callback validator is used internally, by the CreditCard validator, this also affects the signature of the service option to include context and declares the $context param in CreditCard::isValid() so that it can correctly be passed to the composed callback validator.

Will need to document this change in #253

Edit: a bit more clarity…

Previously, the callback might receive (mixed $value, ...$userDefinedArgs) or (mixed $value, array $context, ...$userDefinedArgs) depending on whether the context was given to Callback::isValid() as the second param. This is difficult to document and hard to understand when the given arguments might differ.

This change means that the required signature of the callback is consistent at all times, but $context might be === [] in standalone usage.

[gsteel]

Partially addresses #96