ApiView feedback: custom code serialization abstraction

[trrwilson]

This does the standalone changes previous incorporated into #42 that address the ApiView feedback of extreme repetition in the needed custom serialization code.

It establishes a set of templated static helpers along with a common pattern of SerializeFoo/DeserializeFoo that are then passed as delegates into those helpers. This allows full flexibility with derived type partial (de)serialization relative to instance-only methods.

Together, this makes the common impl pattern fairly minimal:

public partial class Foo : IJsonModel<Foo>
{
    Foo IJsonModel<Foo>.Create(ref Utf8JsonReader reader, ModelReaderWriterOptions options)
        => ModelSerializationHelpers.DeserializeNewInstance(this, DeserializeFoo, ref reader, options);

    Foo IPersistableModel<Foo>.Create(BinaryData data, ModelReaderWriterOptions options)
        => ModelSerializationHelpers.DeserializeNewInstance(this, DeserializeFoo, data, options);

    void IJsonModel<Foo>.Write(Utf8JsonWriter writer, ModelReaderWriterOptions options)
        => ModelSerializationHelpers.SerializeInstance(this, SerializeFoo, writer, options);

    BinaryData IPersistableModel<Foo>.Write(ModelReaderWriterOptions options)
        => ModelSerializationHelpers.SerializeInstance(this, options);

    string IPersistableModel<Foo>.GetFormatFromOptions(ModelReaderWriterOptions options) => "J";

    internal static Foo DeserializeFoo(JsonElement element, ModelReaderWriterOptions options = default)
    {
        // Deserialize from element here, can easily call derived type methods as needed
    }

    internal static void SerializeFoo(Foo foo, Utf8JsonWriter writer, ModelReaderWriterOptions options)
    {
        // Serialize "foo" here, can invoke overriden derived type serializers like "WriteDerived" as needed
    }

    // As needed for type hierarchies
    internal abstract void WriteDerived(Utf8JsonWriter writer, ModelReaderWriterOptions options);
}

[trrwilson]

Closing in favor of
#113