Fix package url for grpc integration

[jaeheonji]

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Description

When I followed the example code exactly, I saw an error in the application because of the wrong import path.

I'm not sure about the history, but it seems that the stub code of protobuf has been moved from dapr/dapr to dapr/go-sdk.
Since UnimplementedAppCallbackServer is imported from dapr/go-sdk, common should also be imported from same package.

[github-actions]

Stale PR, paging all reviewers

[yaron2]

Thanks for your contribution!