Making relations update/remove opt-in instead of opt-out

[TriPSs]

It's currently already placed inside the code as a TODO but wanted to check what others think about this.

Things that I notice in our own API's is that in almost all relations we set disableUpdate and disableRemove, recently the update.<options> and remove.<options> (#123) where added to also support relations decorators, descriptions etc.

My main idea is to align the update/remove with the aggregate and make it opt-in, this would change then from this:

@Relation('todoItem', () => TodoItemDTO, { disableRemove: true })

To this:

@Relation('todoItem', () => TodoItemDTO, { 
   update: { enabled: true }
})

[TriPSs]

cc: @GP4cK @smolinari

[GP4cK]

I'm good with that too.

[smolinari]

LGTM. 😁

Scott