Add feature toggle for Model registry

Add a feature toggle for Model registry so that users can control visibility for GitLab Model registry, similar to what was done previously for model experiments. Mock-up for reference:

There are several steps involved in adding this toggle. Example MRs listed for reference:

• Add configuration to project and database - Adds backend project setting for model registry (!138399 - merged)
  • Example: Adds model for model_experiments_access_level (!121650 - merged)
• Update read and write policies permissions policies - Adds project feature availability check for mod... (!138995 - merged)
  • Examples:
  • Creates policy for model experiments access (!121654 - merged)
  • Adds write policy for Model experiments (!126041 - merged)
• Add API endpoint to control project setting - Adds API to control model registry project setting (!139004 - merged)
  • Example: Resolve "Add API for enabling/disabling model e... (!132972 - merged)
• Add frontend UI
  • Example: Adds frontend to toggle visibility of model exp... (!121653 - merged)