Add new mapTransform function

[prayansh]

In our efforts to provide convenience APIs, we have added a new function JsonObject.mapTransform(Map<String, String>,((String, JsonElement) -> JsonElement)?) that allows you to easily map keys and transform values based on a closure. Example usage:

 properties.mapTransform(mapOf("x" to "y")) { newKey, value ->
    var newVal = value
    if (newKey == "y") {
        // ... do something
    }
    newVal
}