GH-1245 Add initial proxy support

This feature woudl allow request to be proxied to additional targets such as grpc, rsocket etc.

Resolves #1245

spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/FunctionProperties.java

@@ -60,6 +60,11 @@ public class FunctionProperties implements EnvironmentAware, ApplicationContextA
 	 */
 	public final static String FUNCTION_DEFINITION = PREFIX + ".definition";
 
+	/**
+	 * Key for the proxy name.
+	 */
+	public final static String PROXY = "proxy";
+
 	/**
 	 * Definition of the function to be used. This could be function name (e.g., 'myFunction')
 	 * or function composition definition (e.g., 'myFunction|yourFunction')