Provide a gRPC endpoint to getting the version

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Currently, we only have one endpoint for running the test cases. The [VS Code extension](https://github.com/LinuxSuRen/vscode-api-testing) could communicate with it. It would be great if the VS Code side could get the version and prompt users if they upgrade `atest` according to the version comparison.

Please feel free to read the following references before trying to fix this issue:

* https://github.com/LinuxSuRen/api-testing/blob/master/pkg/server/server.proto
* https://grpc.io/docs/languages/go/quickstart/

Start with pkg/server/server.proto to understand the existing gRPC service and its test-case endpoint, then consult the Go gRPC quickstart linked in the issue. Add a version endpoint that the VS Code extension can call and compare, and verify that the service exposes the version successfully.

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