Add a protobuf item template

[JunTaoLuo]

Addresses #9764.

I've verified it on the command line but I'm still figuring out how to test it in VS.

FYI, CLI help:

(AspNetCore) C:\gh\tp\Grpc> dotnet new --list
Usage: new [options]

Options:
...

Templates                                         Short Name            Language          Tags
-------------------------------------------------------------------------------------------------------------------------------
...
ASP.NET Core gRPC Service                         grpc                  [C#]              Web/gRPC
global.json file                                  globaljson                              Config
NuGet Config                                      nugetconfig                             Config
Dotnet local tool manifest file                   tool-manifest                           Config
Web Config                                        webconfig                               Config
Solution File                                     sln                                     Solution
Protocol Buffer File                              proto                                   Web/gRPC

(AspNetCore) C:\gh\tp\Grpc> dotnet new proto --help
Usage: new [options]

Options:
  -h, --help          Displays help for this command.
  -l, --list          Lists templates containing the specified name. If no name is specified, lists all templates.
  -n, --name          The name for the output being created. If no name is specified, the name of the current directory is used.
  -o, --output        Location to place the generated output.
  -i, --install       Installs a source or a template pack.
  -u, --uninstall     Uninstalls a source or a template pack.
  --nuget-source      Specifies a NuGet source to use during install.
  --type              Filters templates based on available types. Predefined values are "project", "item" or "other".
  --dry-run           Displays a summary of what would happen if the given command line were run if it would result in a template creation.
  --force             Forces content to be generated even if it would change existing files.
  -lang, --language   Filters templates based on language and specifies the language of the template to create.


Protocol Buffer File
Author: Microsoft
Description: A protocol buffer file for describing messages and services for gRPC.
    (No Parameters)

[JamesNK]

Looks fine to me. Double check with @shirhatti on content and a template expert on implementation

[JunTaoLuo]

I'm going to count @shirhatti's comment as an approval. @ryanbrandenburg any thoughts on the changes here?

[JunTaoLuo]

Test failure due to https://github.com/aspnet/AspNetCore-Internal/issues/2512. Didn't rebase on the fix but it should be resolved.