Add a `Snapshot` suffix to `SensorState` and `ElectricalComponentState`

[llucax]

What's needed?

SensorState and ElectricalComponentState represent a view of the state at some particular sampled_at time. State are not exactly the same as samples, as samples are usually numeric, but state is a fixed set of possibilities.

Proposed solution

Rename SensorState and ElectricalComponentState to SensorStateSnapshot and ElectricalComponentStateSnapshot respectively, and also rename sampled_at to snapshot_time in both.

[thomas-nicolai-frequenz]

Rename SensorState and ElectricalComponentState to SensorStateSample and ElectricalComponentStateSample respectively.

No we discussed that and this is not a sample! We will rather rename sampled_at. Thats in the making. We are reporting states not metrics!

[llucax]

OK, according to what I've seen in #329 and some talks, I guess the decision is to rename to XxxState -> XxxStateSnapshot and rename sampled_at -> snapshot_time. I will update the issue accordingly.

[tiyash-basu-frequenz]

@ktickner suggests origin_time instead of snapshot_time.