scripts: runners: nrf: Add support for multiple nRF backends

Generalize the logic that was previously in nrfjprog.py into a new
nrf_common.py, which can then use specific tool subclasses, one of which
is nrfjprog.py, to implement the actual tool invocation.

This commit lays down the groundwork for the addition of a new backend
for nRF boards, the new nrfutil tool from Nordic. Both nrfjprog and
nrfutil will coexist in the immediate future, but nrfutil is expected to
be the sole tool for everything nRF in due time.  This is why the internal
representation of the operations is based in JSON and is using the exact
format that nrfutil expects when using the 'execute-batch' option, that
takes a JSON file with an array of operations to be executed.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>