Prototype support for handling absence

[ZacSweers]

(edit: see following comments, this was generalized away from just Optional)

This is a prototype of what Optional support would look like. This implements it in two parts:

1. A new OptionalJsonAdapter + associated factory. This essentially works by associating nulls with Optional.absent() and used for properties that are either present or explicitly null.
2. Reflective support in ClassJsonAdapter and KotlinJsonAdapter for supporting defaulting absent values to Optional.empty().

Open questions:

1. Do we really want to support this in Kotlin? If so, how do we handle the following:
   a. Optional<String>?
   b. Do we support this in code gen too?
2. Do we support just j.u.Optional or do we want to do something more generic (to support Guava, KOptional, etc)? How would that look if so?
3. Do we want to generify absent properties in general? Maybe an optional JsonAdapter.ifAbsent() or separate AbsenceHandler interface?

• This would obviate 1 and 2 as we just need to support this API and delegate to whatever adapters folks bring.

[ZacSweers]

Pushed an absence handler API demo instead, which I think came out pretty clean all things considered!

[ZacSweers]

Should say I'm not thrilled about the of adding this directly to the JsonAdapter API, but without a clear/easy way to access delegated adapters it's not clear how to unpack an opt-in interface's APIs

[lassem]

In Kotlin I'd use this to fall back to any default arguments on data classes. In our existing implementation this is achieved via a work around utilising a custom @DefaultIfNull annotation and deserialising via Map first, which of course is additional overhead.

[ZacSweers]

@lassem we already do that in kotlin. This would only be for Java or for kotlin cases where you don't provide a default and simply want it to coerce to Optional.empty()

[lassem]

That is true. However, the aforementioned @DefaultIfNull annotation would also use the default if the json specifically said null

[ZacSweers]

I don't see how it would since an absent key would not be present in a map. In any case let's keep this PR on topic