gRPC: Document strategies for sharing proto files between apps

> Copy the Protos\greet.proto file from the gRPC Greeter service to the gRPC client project.

No, really, please just say how the client can utilize the same service definition or its like representation - without requiring that we maintain two or more copies of the definition. Every other web service has had this kind of ability (wsdl, wcfmex/service contract, swagger/openapispec, etc). This tutorial should be showing us noobs the _correct_ and practical (without having to rely on object remappers?) way.

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* ID: 1d298a6c-8ddc-dbc8-d219-0fa7138442c2
* Version Independent ID: b96be346-958a-cfcd-d80a-c352d64223e8
* Content: [Create a .NET Core gRPC client and server in ASP.NET Core](https://docs.microsoft.com/en-us/aspnet/core/tutorials/grpc/grpc-start?view=aspnetcore-5.0&tabs=visual-studio)
* Content Source: [aspnetcore/tutorials/grpc/grpc-start.md](https://github.com/dotnet/AspNetCore.Docs/blob/main/aspnetcore/tutorials/grpc/grpc-start.md)
* Product: **aspnet-core**
* Technology: **aspnetcore-tutorials**
* GitHub Login: @JunTaoLuo
* Microsoft Alias: **johluo**