GEP: Replace Certificate Refs on HTTPRoute with Cross Namespace Refs from Gateway

[robscott]

What would you like to be added:
This proposes replacing Certificate Refs on HTTPRoute with the ability for Gateways to reference certificates in other namespaces. This is not a complete replacement, but it does significantly reduce the potential for confusion.

Why this is needed:
This is a confusing part of the API that does not appear to be supported by any implementation yet. We can always choose to add this concept back if it's necessary, but leaving it in for v1alpha2 may leave us stuck with this feature for a long time.

This is a follow up on my original proposal doc.