Add API for enabling/disabling model experiments

Disabling Model experiments in Packages and reg... (#412384 - closed) added an UI for enabling/disabling model experiments with Adds frontend to toggle visibility of model exp... (!121653 - merged). The project model was extended with Adds model for model_experiments_access_level (!121650 - merged).

The projects API was however not augmented with equivalent functionality.

We configure our GitLab projects through the API and not the UI and would like to enable/disable model experiments.

I don't know if there's some official GitLab policy on UI contra API feature disparity but this seems to me like an omission that should be prioritized.