Support C# 11 required modifier in Grpc.Tools

[lukedukeus]

C# 11 adds properties that can be marked as required - https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/required.

When a property is required, it must be set in the object initializer. If a protobuf message does not mark a property as optional, it should be required in C#. Right now, I can do:

TestMessage.proto:

syntax = "proto3";

message TestMessage {
    int32 non_optional_property = 1;
    optional int32 optional_property = 2;
}

Test.cs:

TestMessage test= new TestMessage()
{

};

And I don't get any build errors. Instead, what I would expect is:
Required member 'TestMessage.NonOptionalProperty' must be set in the object initializer or attribute constructor.

Additional context

Very similar to 20729