Introduce support for interacting with a running instance of Mixe

[geeknoid]

We want the ability to dynamically inspect and control a running instance of Mixer and other Istio components. This for things like inspecting counters, controlling log levels, tweaking various parameters, etc.

This functionality has two uses:

• Aid in developing and debugging Mixer
• Aid in operating Mixer, to track down failures or performance issues

As such, we want a user experience that is friendly to operators.

This issue tracks the creation of a proposal and its eventual implementation.