Migrates token relay from spring-cloud-gateway-server-security.
Migrates token relay from spring-cloud-gateway-server-security.

It moves it to spring-cloud-gateway-server

See gh-1975

spring-cloud-incubator/spring-cloud-gateway,spencergibb/spring-cloud-gateway,spencergibb/spring-cloud-gateway,spring-cloud-incubator/spring-cloud-gateway