Callback on parameter value update

[kylestach]

Description

We should be able to run callbacks (like switching a network port) to parameter change events.

Requirements

• Allow attaching callbacks to parameter update events

Design

This can be added in a straightforward manner to the parameter provider (by adding a callback to call when Update() is hit for a particular parameter). However, the manner by which to register parameters still needs design clarification. PARAM_* objects are primitive types, so it can't be added directly to them, but we could pass in a "param handle" in the form of a parameter name string or a pointer to a variable.