Allow configuration of `relativeTo` in the `routerLink` directive. This
is related to the clearing of auxiliary routes, where you need to use
`relativeTo: route.parent` in order to clear it from the activated
auxiliary component itself. This is because `relativeTo: route` will
consume the segment that we're trying to clear, so there is really no
way to do this with routerLink at the moment.
Related issue: #13523
Related (internal link): https://yaqs.corp.google.com/eng/q/5999443644645376
PR Close#39720
112324a614