Old PR redux: helpers for common .Serialization.cs patterns, header cleanup

[trrwilson]

This PR is a contemporary followup to the earlier one -- better now that we're fully aligned with a codegen base:
#48

The helper still brings some utility in centralizing logic and eliminating duplication in the custom layer.

It also opportunistically fixes a lot of ugly copy/paste header errors in the Assistants folder.

To rehash the helper PR (longer description there):

• Custom classes should implement an internal static Foo DeserializeFoo(JsonElement, ModelReaderWriterOptions) (as before)
• Now, they should also implement a matching internal static void SerializeFoo(Foo, Utf8JsonWriter, options)
• All of the IJsonModel<Foo> implementations can then just defer to the CustomSerializationHelpers methods, allowing everything but the serialize/deserialize methods to be one trivial line
• The helpers also centralize format/error checking and other repeated patterns, reducing overall line count and "noise"