Pass in interpolation functions to ParticleSystem

Currently, particles will always linearly interpolate between `startColor` and `endColor`, and `startScale` and `endScale`. I want the ability to pass in a function for each to compute the interpolation. The default value would still be `CesiumMath.lerp`.

Start by locating the ParticleSystem entry point and read how it currently interpolates startColor/endColor and startScale/endScale. Determine where interpolation functions would be passed and how CesiumMath.lerp remains the default. The work is done when both properties accept custom functions while preserving the existing linear behavior.

Written by the indexing model from the issue text.