Create a single runSimulations function to be used in both async and sync context for all run models

[xjules]

Currently all run models have two run functions; one running the async version (eg. https://github.com/equinor/ert/blob/e15c1101e70d150d5a105b50f18300d5013a7bd6/src/ert/shared/models/ensemble_experiment.py#L30) and one running the sync version (eg. https://github.com/equinor/ert/blob/e15c1101e70d150d5a105b50f18300d5013a7bd6/src/ert/shared/models/ensemble_experiment.py#L117). As suggested here it would be beneficial to have just a single version that runs both.