feat: add isExternal argument to simplify ember-engines

[BobrImperator]

• Adds isExternal argument to LinkTo component
  • based on the flag it disables assertions that are causing Routeless engines to fail, ember-engines internally overrides them.
  • based on the flag, it changes the behavior of namespaceRoute method, so it uses the provided route directly without changing it further - in that case ember-engines is responsible for passing correctly resolved routes.

[SergeAstapov]

@BobrImperator FYI this contradicts the long term and more stable solution proposed in https://github.com/emberjs/rfcs/pull/819

[kategengler]

Closing since it contradicts the RFC