[Step2|BE] Add allowed actions to SnippetInputAction

In #217795 (closed) and #217799 (closed) we are going to pass actions to their respective mutations.

Nevertheless, the actions despite of being valid, depending on the context they can be acceptable or not. For example, when we create a snippet through the mutation, it doesn't make sense to have :delete, :update, or :move actions, only the :create action make sense there.

Therefore, we need a way to tell the SnippetInputAction class what are the allowed actions that we permit.