[RFR] Discriminated union type for auth action

[pemoreau]

Applied discriminated union type for auth actions to make the code more uniform

[djhi]

Can you make sure we export the type from auth ?

Users should be able to import them from ra-core

[pemoreau]

Thanks.
I think this is the case:

• AuthActionType is exported by src/auth/types.ts
• src/auth/index.ts contains export * from './types';
• src/index.ts contains export * from './auth';
  but correct me if I do not see something