Support HTTP GET of single value value Function

E.g. a Function<Long, Foo> can be used to fetch a single entity of
type Foo with a long ID., via /{function}/{id}

spring-cloud-function-web/src/main/java/org/springframework/cloud/function/web/FunctionController.java

@@ -79,4 +79,15 @@ public class FunctionController {
 		Flux<String> result = (Flux<String>) supplier.get();
 		return debug ? result.log() : result;
 	}
+
+	@GetMapping(path = "/{name}/{value}")
+	public Flux<String> value(@PathVariable String name, @PathVariable String value) {
+		Function<Flux<?>, Flux<?>> function = functions.lookupFunction(name);
+		if (function != null) {
+			@SuppressWarnings({ "unchecked" })
+			Flux<String> result = (Flux<String>) function.apply(Flux.just(value));
+			return debug ? result.log() : result;
+		}
+		throw new IllegalArgumentException("no such function: " + name);
+	}
 }

spring-cloud-function-web/src/test/java/org/springframework/cloud/function/web/RestApplicationTests.java

@@ -150,6 +150,16 @@ public class RestApplicationTests {
 				.isEqualTo("[FOO][BAR]");
 	}
 
+	@Test
+	public void uppercaseGet() {
+		assertThat(rest.getForObject("/uppercase/foo", String.class)).isEqualTo("[FOO]");
+	}
+
+	@Test
+	public void convertGet() {
+		assertThat(rest.getForObject("/wrap/123", String.class)).isEqualTo("..123..");
+	}
+
 	@Test
 	public void uppercaseJsonArray() throws Exception {
 		assertThat(rest.exchange(
@@ -193,6 +203,11 @@ public class RestApplicationTests {
 					.map(value -> "[" + value.trim().toUpperCase() + "]");
 		}
 
+		@Bean
+		public Function<Flux<Integer>, Flux<String>> wrap() {
+			return flux -> flux.log().map(value -> ".." + value + "..");
+		}
+
 		@Bean
 		public Function<Flux<HashMap<String, String>>, Flux<Map<String, String>>> maps() {
 			return flux -> flux.map(value -> {