Canonical: Introduce `x_redirect_by` filter that allows applications to...

Canonical: Introduce `x_redirect_by` filter that allows applications to identify themselves via `X-Redirect-By` header when they're doing a redirect.

Props joostdevalk.
Fixes #42313.
Built from https://develop.svn.wordpress.org/trunk@42408


git-svn-id: https://core.svn.wordpress.org/trunk@42237 1a063a9b-81f0-0310-95a4-ce76da25c4cd