proxy(router): Allow to set some outgoing headers

This introduces the "additional_headers", "remote_user_header" and
"skip_x_access_token" config keys to allow configuring routes to
external services that require addtional headers to be set.

"remote_user_header": defines the name of a Header that will carry the
userid of the authenticated user on the outgoing request.

"additional_headers": defines a list of header names and values that will
be added to outgoing requests on matching routes.

"skip_x_access_token": when set to true the reva access token will not
be added to the outgoing request.

Needed for #206

services/proxy/pkg/router/router.go

@@ -86,9 +86,11 @@ func New(serviceSelector selector.Selector, policySelectorCfg *config.PolicySele
 
 // RoutingInfo contains the proxy rewrite hook and some information about the route.
 type RoutingInfo struct {
-	rewrite     func(*httputil.ProxyRequest)
-	endpoint    string
-	unprotected bool
+	rewrite          func(*httputil.ProxyRequest)
+	endpoint         string
+	unprotected      bool
+	remoteUserHeader string
+	skipXAccessToken bool
 }
 
 // Rewrite returns the proxy rewrite hook.
@@ -101,6 +103,17 @@ func (r RoutingInfo) IsRouteUnprotected() bool {
 	return r.unprotected
 }
 
+// RemoteUserHeader returns the name of Header for setting the remote user value
+func (r RoutingInfo) RemoteUserHeader() string {
+	return r.remoteUserHeader
+}
+
+// SkipXAccessToken return true if the reva access token should not be added to the
+// outgoing request
+func (r RoutingInfo) SkipXAccessToken() bool {
+	return r.skipXAccessToken
+}
+
 // Router handles the routing of HTTP requests according to the given policies.
 type Router struct {
 	logger          log.Logger
@@ -126,8 +139,10 @@ func (rt Router) addHost(policy string, target *url.URL, route config.Route) {
 	}
 
 	rt.rewriters[policy][routeType][route.Method] = append(rt.rewriters[policy][routeType][route.Method], RoutingInfo{
-		endpoint:    route.Endpoint,
-		unprotected: route.Unprotected,
+		endpoint:         route.Endpoint,
+		unprotected:      route.Unprotected,
+		remoteUserHeader: route.RemoteUserHeader,
+		skipXAccessToken: route.SkipXAccessToken,
 		rewrite: func(req *httputil.ProxyRequest) {
 			if route.Service != "" {
 				// select next node
@@ -161,6 +176,10 @@ func (rt Router) addHost(policy string, target *url.URL, route config.Route) {
 				req.Out.Host = target.Host
 			}
 
+			for k, v := range route.AdditionalHeaders {
+				req.Out.Header.Set(k, v)
+			}
+
 			req.Out.URL.Path = singleJoiningSlash(target.Path, req.Out.URL.Path)
 			if targetQuery == "" || req.Out.URL.RawQuery == "" {
 				req.Out.URL.RawQuery = targetQuery + req.Out.URL.RawQuery