feat: validate actions configuration definition

[realSpok]

Definition of Done

General

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

Security

• [ ] Consider the security impact of the changes made

[Thenkei]

You should probably change the packages/forestadmin-client/src/schema/types.ts#ForestServerActionField too.

[Thenkei]

What about old widget definition?

widget: null | 'belongsto select' | 'file picker';

[realSpok]

What about old widget definition?

It will be updated in another PR