Fix PMBus backend selector

[mkeeter]

(There's a new big block comment that explains what's going on, so you may want to go read that)

Previously, we would always select the IdolWorker PMBus agent when network connected. This is fine for the udprpc backend, where the HIF program is executed on the host computer and so the additional overhead of Idol IPC doesn't matter.

However, the introduction of the hiffy+net backend means that we would (1) select the IdolWorker agent, because the system is network-connected then (2) use the hiffy backend, executing the larger programs on the SP. This is not ideal!

This PR teaches the pmbus crate about the different HiffyContext backends, so that it can make smarter choices.

[jamesmunns]

Made a suggestion for a different way to organize this in #704, feel free to take it or leave it :)

I think the current change/comment is reasonable to understand as-is, though my personal style pref is #704.