docs: add MessagePack release notes and expand README with XElement/Binary limitations

[Copilot]

Adds documentation for the MessagePack transport preview (5.10.0 / AspNetCore 1.6.0), covering server and client setup, custom serializer injection, and known type limitations.

Changes

• Changelog.md — new 5.10.0 / AspNetCore 1.6.0 section at the top covering:

  • Server-side AddMessagePackSerialization() setup
  • Client-side MessagePackHttpDomainClientFactory setup
  • Custom serializer injection on both sides
  • Link to PR Add new transport based on MessagePack #591 for benchmark data

• src/OpenRiaServices.Hosting.AspNetCore/Framework/README.md — expanded the Supporting MessagePack wire format section with:

  • Known limitations: XElement and System.Data.Linq.Binary have no built-in converters and must be handled explicitly
  • Links to reference implementations (XElementConverter, BinaryConverter) in the test website
  • Code snippet showing how to register custom converters via the options callback:

builder.Services.AddOpenRiaServices()
    .AddMessagePackSerialization(opt =>
    {
        opt.Serializer = new Nerdbank.MessagePack.MessagePackSerializer()
        {
            Converters = [new XElementConverter(), new BinaryConverter()],
        };
    });