Add Client Order ID field for ChannelAdvisor

[ja]

We currently use the ChannelAdvisor Order ID as our ShipStation Order number, but we still need a way to access the original channel order ID (such as Amazon order number, Shopify order number, etc.) to print on packing slips and send in email notifications to customers. This field is called the "Client Order #" in the ChannelAdvisor connection settings page.

Currently, we can choose between "Channel Advisor Order #" and "Client Order #" when setting up the connection between ShipStation and ChannelAdvisor. This means both fields are available in the connection between ShipStation and ChannelAdvisor.

We can also request that ShipStation support map the "Channel Advisor Order #" field to a custom field in ShipStation. Unfortunately, this support mapped feature is not available for the "Client Order #" field. 

It seems like this should be fairly straightforward to implement, and it would have a massive impact on our ability to use ShipStation for shipping notifications. Ideally we would have this available as a secondary order ID field, but just mapping it to a custom field would be a big win!