Add type field to action creators

[chawes13]

This will allow for better RTK support when defining apiActions in the extraReducers field.

The toString() override is getting removed in 2.0 in favor of referencing the type attribute.

Instead of having to write

extraReducers: (builder) => {
  builder.addCase(apiActions.fetchTodo.toString(), todoHandler)
}

we can add a type attribute and simply pass in the action creator function itself

extraReducers: (builder) => {
  builder.addCase(apiActions.fetchTodo, todoHandler)
}

Personally, I forgot to do this several times and it led to some pretty subtle bugs