[Enhancement] Extraction of constants [example module grpc http directory]

[Alonexc]

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• [X] I had searched in the issues and found no similar issues.

Enhancement Request

Describe the solution you'd like

Located at:eventmesh-examples/src/main/java/org/apache/eventmesh/grpc and http.demo Extract these constants from the image and put them in the ExampleConstants class in the common directory, and change all the classes that use these constants.

Are you willing to submit PR?

• [ ] Yes I am willing to submit a PR!