GH-247 Added spring.cloud.function.definition property

Added spring.cloud.function.definition property which is used by FunctionRegistry as a supplement instruction to resolve nameless lookups.
It is used by web module to map single or multiple (composed) functions to the root path (/)

Resolves #247

spring-cloud-function-context/src/main/resources/META-INF/spring-configuration-metadata.json

@@ -7,6 +7,12 @@
         "type": "java.lang.String",
         "description": "Triggers scanning within the specified base packages for any class that is assignable to java.util.function.Function. For each detected Function class, a bean instance will be added to the context.",
         "defaultValue": "functions"
+    },
+    {
+        "name": "spring.cloud.function.definition",
+        "type": "java.lang.String",
+        "description": "Name (e.g., 'foo') or composition instruction (e.g., 'foo|bar') used to resolve default function especially for cases where there is more then once function available in catalog.",
+        "defaultValue": ""
     }
   ]
 }