Feat: Add support for computed values in sensors / parameters

The MG4 is quite a quirky car as it doesn't expose power values directly, instead it relies on voltage and amperage sensors to get the proper value (even when charging).

It would be nice to have a way of specifying "combine those two" to get the actual power out.

No files, tests, or entry points are named. Start by tracing how sensors and parameters are defined and how voltage and amperage values are exposed, then determine where a computation mechanism could combine them into power values; done means the MG4's computed power is available, including while charging.

Written by the indexing model from the issue text.