Adding a Kotlin Spring Boot example

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[jamesward]

Thanks! Can you do that CLA thing?

[joshlong]

Thanks! Can you do that CLA thing?

Done and done

[jamesward]

Thanks! I cleaned up a bit: https://github.com/GoogleCloudPlatform/cloudbowl-microservice-game/commit/5679ed7730e6d1f1c5e4d97bf266858844dfe850

Added to the test arena: https://cloudbowl.gcplab.me/test