Checkout service integration

[nicolehaugen]

Need to integrate the checkout service with auth\api gateway and also hook it up to use the existing client.

In addition, need to add docker files.

[chlowell]

The checkout service also needs to produce Kafka messages for the sticker service when orders are created. I spent some time on this with Confluent.Kafka but couldn't get it to communicate with the broker. For now I'll assign myself for tracking and punt this to future me, or anyone who wants to play around with it in the meantime.

[nicolehaugen]

I opened a new issue to track the Kafka piece for this (#53) - closing this issue since the original work item has been completed and merged into master.