Add Google Protocol Buffers serialization documentation for Orleans

[Copilot]

Summary

Adds documentation for the Microsoft.Orleans.Serialization.Protobuf package to Orleans serialization configuration and customization guides.

Fixes #44791

Changes

• serialization-configuration.md: Added "Configure Orleans to use Google Protocol Buffers" section with installation and configuration instructions using AddProtobufSerializer
• serialization-customization.md: Updated serialization providers list from two to three implementations, including Protobuf

Usage Example

siloBuilder.Services.AddSerializer(serializerBuilder =>
{
    serializerBuilder.AddProtobufSerializer(
        isSupported: type => type.Namespace.StartsWith("Example.Namespace"));
});

Documentation follows the existing pattern established by Newtonsoft.Json and System.Text.Json sections.

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>Orleans: add details about Google Protocol Buffers to serialization docs</issue_title>
<issue_description>### Describe the issue or suggestion

Add details of the google protocol buffers support package to these docs:

• https://learn.microsoft.com/en-us/dotnet/orleans/host/configuration-guide/serialization-configuration

• https://learn.microsoft.com/en-us/dotnet/orleans/host/configuration-guide/serialization-customization?pivots=orleans-7-0

The package is here, alongside its README: https://www.nuget.org/packages/Microsoft.Orleans.Serialization.Protobuf
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Orleans: add details about Google Protocol Buffers to serialization docs #49926

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