Resource Server should support WebClient Bearer Token propagation

[jzheaux]

A resource server can propagate the bearer token by fashioning an ExchangeFilterFunction:

ExchangeFilterFunction oauth2 = (request, next) -> {
    AbstractOAuth2TokenAuthenticationToken<?> authentication = 
        (AbstractOAuth2TokenAuthenticationToken<?>) 
            SecurityContextHolder.getContext().getAuthentication();
    request.headers().setBearerAuth(authentication.getToken().getTokenValue());
    return next.exchange(request);
};

It would be nice to remove some of this boilerplate.

[jzheaux]

Related to https://github.com/spring-cloud/spring-cloud-security/issues/217

[jgrandja]

@jzheaux Ticket #5334 is related as well. Please consider a solution that could resolve both issues.