Clamp density function takes a direct input and doesn't allow a reference

The "clamp" density function type has an "input" field. But it only allows a direct (inlined) density function or a constant, it doesn't allow a reference (string). This is inconsistent with every other density function that takes another density function as argument.

The reason is because DensityFunctions.Clamp uses DensityFunction.DIRECT_CODEC, instead of DensityFunctions.HOLDER_HELPER_CODEC, which is what every other density function type uses.