Add `_voltage` suffix to `primary`/`secondary` in `PowerTransformer`

[llucax]

What's needed?

Now that VoltageTransformer was renamed to PowerTransformer, it is not all that clear what primary and secondary is about. It would be good to make it more explicit.

Proposed solution

Rename primary -> primary_voltage and secondary -> secondary_voltage.

[tiyash-basu-frequenz]

If this PR gets merged #440, we could change the PowerTransformer message to the following:

message PowerTransformer {
    TransformerRatio voltage_transformer_ratio = 1;
}