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