feat: implement FOREST_DISABLE_AUTO_SCHEMA_UPDATE and test for it

[gonzalomolbar]

Definition of Done

• Added support for a new optional setting FOREST_DISABLE_AUTO_SCHEMA_UPDATE which, if true, will stop the forestadmin-schema.json to be overwritten but still generating schema data for the app to function properly
• It only applies in the scenario where the file is written, i.e. if DEBUG = true
• Wrote test for it, following standards
  • Corrected a few usages of self.assertRaises in other tests in the file wich were not being used properly

Note

I didn't add anything in the changelog or versioning because I'm less familiar with it, and I'm not sure if you're the ones to add those changes when releasing it! So let me know if I should add anything

General

• [x] Write an explicit title for the Pull Request, following Conventional Commits specification
• [x] Test manually the implemented changes
• [x] Validate the code quality (indentation, syntax, style, simplicity, readability)

Security

• [x] Consider the security impact of the changes made