Update serde interfaces to differentiate in/out

[JordonPhillips]

This updates the interfaces for rest services to differentiate when an operation's input vs output vs error is being serialized / deserialized. This is necessary because there can be subtly different rules in each case.

Right now this is pretty ugly so I put it up as a draft. The major alternative I am considering is to instead have explicit methods for each case, e.g:

protected abstract void serializeInputDocumentBody(
    GenerationContext context,
    OperationShape operation,
    List<HttpBinding> documentBindings
);

protected abstract void serializeOutputDocumentBody(
    GenerationContext context,
    OperationShape operation,
    List<HttpBinding> documentBindings
);

protected abstract void serializeErrorDocumentBody(
    GenerationContext context,
    OperationShape operation,
    StructureShape error,
    List<HttpBinding> documentBindings
);

Implementers could then make it generic on their end if necessary as apposed to having to split things up like is the case now.

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[adamthom-amzn]

I don't like adding a boolean flag to methods in general, but looking at the structure of methods like writeContentTypeHeader I think you may end up having a private method with the flag added anyway.

[JordonPhillips]

rebased so tests can be run one more time