optimize dubbo-triple-demo

[heliang666s]

What is the purpose of the change?

Although the original demo used the proto file, the Service interface was still defined, which seems to be not very standardized, so I moved the definition of the service interface to the proto file.

Checklist

• Make sure there is a GitHub_issue field for the change.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit-test to verify your logic correction. If the new feature or significant change is committed, please remember to add sample in dubbo samples project.
• Make sure gitHub actions can pass. Why the workflow is failing and how to fix it?