Adds list catalog method to Square facade

[Mrkbingham]

This adds a method to retrieve catalog data from the Catalog API, allowing users to pass in non (returns all) one or many catalog types.

There are three tests added:

1. One that confirm the entire catalog can be retrieved
2. One that confirms the request can be filtered to specific items
3. One that validates exceptions will be thrown when filtering incorrect items.

Usage
This new method can be used by calling:
Square::listCatalog()
and optionally can take in an array of types:
Square::listCatalog(['ITEM'])

Although it's recommended you use the CatalogObjectType model to prevent issues that arise when using strings:

use Square\Models\CatalogObjectType;

$catalogItems = Square::listCatalog([CatalogObjectType::ITEM]);