Add display value UI hints for sensors

Adds a new `RenderingHints.DisplayValue` class that implements a transformer `Function` that is applied to an `AttributeSensor` value when it is rendered in the UI. The `UsesJavaMXBeans.USED_HEAP_MEMORY` and associated sensors are configured to convert the number of bytes from a `Long` to a `String` using the `ByteSizeStrings.metric()` function, so **1000000000** will be shown as **1GB** and so on.