Support dynamic/anonymous properties change

in some contexts the name of a field is not know, we want to support anonymous (runtime) property change, e.g.
using the dynamic? operator from FSharp data or using indexer .[...]

```fsharp
#r "nuget: FSharp.Data.Mutator, 0.1.0-beta"
open FSharp.Data.Mutator
open FSharp.Data
open FSharp.Data.JsonExtensions

type Test = JsonProvider<"""
{ "name" : "hello" }
""">

let sample = Test.GetSample()

sample
|> Change (<@ fun x -> x.["newProperty"] = """{ "test": "hi" }""" @>)

```

at the moment Newtonsoft.Json can be used...