instruments: Add screencap
instruments: Add screencap

Add an instrument which wraps the devlib screencapture poller, which
allows us to capture a screenshot from the device every `period`. The
`period` is configurable based on the user requirements.

All captured screenshots are named according to the timestamp on the
device, and are placed into an output directory (screen-capture)
within the job directory.

lisatn/workload-automation,ARM-software/workload-automation,lisatn/workload-automation,lisatn/workload-automation,setrofim/workload-automation,ARM-software/workload-automation,ARM-software/workload-automation,ARM-software/workload-automation,setrofim/workload-automation,setrofim/workload-automation,lisatn/workload-automation,setrofim/workload-automation