Try to validate that objectives are correctly specified

[oyvindeide]

Should be done after https://github.com/equinor/ert/issues/8847 When a user has an objective, everest expects there to be a file output from the forward model called: objective. It would be good to have an option where Everest can try to validate.

One option is to do this by checking the forward model section for arguments called objective and give a warning as well as a path to resolution, however that is likely to miss a lot of cases, and not be very robust.

Another option is to have some validation hooks where the forward model gets access to the everest objective section and can validate against it.

• For this we could introduce a new forward model type, for example called: objective_function_forward_model, that would get access to the relevant path of the everest config though a plugin hook at startup.
• An example of this is how pytest does it: https://github.com/pytest-dev/pytest-xdist/blob/b05a5bc7b8a86d8496a0a00c4b20fe05dcc27280/src/xdist/plugin.py#L16