Add Additional Namespace & Catalog Request Support

[CATgwalker]

This PR adds a cmdlet for New-ServiceNowCatalogItem which uses the 'sn_sc' API namespace to add a catalog item request to a cart and then submits the cart as an order.

Extends Get-ServiceNowAuth and Invoke-ServiceNowRestMethod with a namespace parameter to support different Namespace endpoints; backwards compatibility is maintained by defaulting to the 'now' namespace if not specified.