Add global option for aliasHeadersStrategy

Welcome!

• Yes, I've searched similar issues on GitHub and didn't find any.
• Yes, I've searched similar issues on the Traefik community forum and didn't find any.

What did you expect to see?

Current config for the new aliasHeadersStrategy is rather inconvenient since it causes a warning for every entryPoint that does not have the setting. This includes eintryPoints "traefik" and "metrics" you don't usually configure at all.

Is there a plan to change the default at some point? Otherwise a global setting would probably be a good idea.