node: export IPMI sensor data

[lorenz]

We want to be able to get all IPMI sensor data as metrics.
There is ipmi_exporter from the prometheus-community org, but it shells out to freeipmi which does not work for us. There is https://pkg.go.dev/github.com/gebn/bmc which contains a pure-Go upper-layer protocol implementation for IPMI, but we need to convert the sensor readings to metrics as well as provide a lower-layer (SMBus) connection implementation.