sensors: get min and max values for sensors

[jamesmunns]

The sensors task holds min and max values received from each sensor. There is an IDL API to get this, and it exists in the core dumps, but we have no easy way to get it.

We should fix that, as it would be useful for things.

[jamesmunns]

These are the specific arrays that would be worth pulling:

https://github.com/oxidecomputer/hubris/blob/b6ba19bcf644bba1b22b3ca8e3e88b8e64795eed/task/sensor/src/main.rs#L64-L68