Add Operation.Data.toJsonString() convenience function for the jvm

[StylianosGakis]

Allows a more convenient way to simply get the Json representation from Operation.Data without having to manually deal with Buffers etc in client code which the current toJson function requires.

This could be imitated on the clients by instead calling the existing toJson function like this:

fun Operation.Data.toJsonString(
    scalarTypeAdapters: CustomScalarAdapters = CustomScalarAdapters.Empty,
    indent: String? = null,
): String {
    val buffer = Buffer()
    val jsonWriter = BufferedSinkJsonWriter(buffer, indent)
    toJson(jsonWriter, scalarTypeAdapters)
    return buffer.readUtf8()
}

Which should be functionally equivalent to what the introduced function does (if we inlined buildJsonString which toJsonString uses internally)

If you believe that the API doesn't need this addition that would be totally understandable, and you can feel free to simply close this PR 🙌

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[martinbonnin]

Thanks!

[BoD]

👍