Adds support for spring-cloud-function and spring-cloud-stream handlers (#3646)

* Add support for spring cloud function handler.

* Add support for spring cloud stream handler.

* Add support for path variable expansion in fn: and stream: URIs

* Adds support for config based fn/stream HandlerFunctions

* Starting support for webflux function support

* Adds fn:functionName support for webflux server

* Adds spring-cloud-starter-stream-rabbit to test scope

* Adds StreamRoutingFilter to webflux server

spring-cloud-gateway-server-mvc/src/test/resources/application-functionhandlerconfigtests.yml

@@ -0,0 +1,16 @@
+spring.cloud.gateway.mvc:
+  routesMap:
+    testsimplefunction:
+      uri: fn:upper
+      predicates:
+        - Path=/simplefunction
+        - Method=POST
+    testtemplatedfunction:
+      uri: fn:{fnName}
+      predicates:
+        - Path=/templatedfunction/{fnName}
+        - Method=POST
+
+logging:
+  level:
+    org.springframework.cloud.gateway.server.mvc: TRACE