Add Shipment Notification Services to ServicesRequest node for FedEx

[jiblits]

As can be seen within this section of the fedex code there's no way to easily tie in to add additional features that some shipping automation may require.

I need to build this anyway for a shipping solution I'm implementing, and if there's interest I could add the feature to mainline.

Information for Shipment Notifications can be found in Section 7 of the Fedex API documentation

What I have in mind:

fedex.build_shipment(origin, destination, package, service_type: service_type,
    notifications: {
        "receiver@example.com": %i(shipment exception),
        "shipper@example.com": %i(exception)
    })

However, if there's something that would be preferred feel free to let me know.

[jiblits]

This is where I've gotten with the feature, which I'll be using for the foreseeable future.

If there's interest in adding it to the main codebase, let me know. Otherwise feel free to close the issue.