Allow installed forward models to supply output format

[oyvindeide]

Currently we have hardcoded which simulators expect to output summary files in:

ert/src/ert/config/summary_config.py

Line 86 in e0d923b

any(sim in _name.lower() for sim in ["eclipse", "flow"])

, however there are other simulators that also do that. Would be good to allow them to specify that through the plugin system so that we can give a better (no) warning depending on the environment they are running in.