Feature: Add `auto` hostname rewrite

[howardjohn]

What would you like to be added:

Today, we can explicitly rewrite the hostname/authority of a request. It would be nice to have an 'auto' mode that derives it from the backend.

For example, consider a case like...

route:
  filter:
    urlRewrite:
      authority: AUTO
  backends
  - google.com
  - wikipedia.org

Why this is needed:

If I only explicitly can set the authority, i cannot do this. I need it to dynamically be set per-backend type.