Add option tagAsContentsFieldName to TaggedObject SumEncoding

[wi11dey]

Hello,

Thanks for making aeson.

I've added a flag tagAsContentsFieldName which will allow encoding sum types as

{
  "tag": "constructorName"
  "constructorName": { ... contents ... }
}

This style is used in type encoding standards such as https://palantir.github.io/conjure/#/docs/spec/wire?id=_54-union-json-format.

Initially I was thinking to make contentsFieldName be a String -> String and provide it the constructor name, which would be flexible enough to allow the default behavior of const "contents" and also be able to dynamically use the constructor name (and even modify it), but this would break backcompat and also not be serializable to the TH implementation.

Please let me know if you'd like any changes to the proposed API