AK2000: enforce immutability of messages

[Aaronontheweb]

All messages should be immutable - and this touches on a number of facets:

1. All properties must be readonly (fields in this case), init-only, or get-only
2. All collection properties must be declared as System.Collections.Immutable, IReadOnly<T>, or the new frozen collections introduced in .NET 8

This one will require a lot more work than the others.

Related Akka.NET issue: akkadotnet/akka.net#3376