Expose the internal entity type readers

[Joe4evr]

After some discussion in #941, I thought that this might be a favorable change.

I decided not to expose the Primitive, Enum, and Nullable readers since there seems to be a bit more complexity involved with those and it seems just too unlikely someone wants to piggyback off of the existing behavior outside of the entities.

[Joe4evr]

Open question: Since after this change, it'll be likely that people inherit one of these readers and call the base implementation to see if that succeeds:

var baseResult = await base.ReadAsync(context, input, services);
if (baseResult.IsSuccess)
{
    //.....
}
return baseResult;

would it be prudent to add a new property to TypeReaderResult that returns the best match provided the result was successful?

public object BestMatch => IsSuccess
    ? (Values.Count == 1 ? Values.Single().Value : Values.OrderByDescending(v => v.Score).First().Value)
    : throw new InvalidOperationException("TypeReaderResult was not successful.");

[Nensec]

Would't it be better that BestMatch simply returned null rather than throw an exception? Doing the IsSuccess check seems more like something the caller should do, plus it prevents doing a simple ?? fallback. Now it requires an entire try{} catch{}

[Joe4evr]

Not really.

1. The caller should check result.IsSuccess before accessing BestMatch (this is the same principle as when handling arbitrary Streams, where you should check CanRead/CanWrite/CanSeek etc. before performing the respective operation).
2. There could be the case where null was actually the value given for that parameter and the caller should respect that.