Only expose Publisher via FunctionCatalog

Flux.from() is cheap and can be used to marshal the inputs everywhere
internally. With this change users ought to be able to register any
function of any Publisher type.

spring-cloud-function-adapters/spring-cloud-function-adapter-aws/src/test/java/org/springframework/cloud/function/adapter/aws/SpringFunctionInitializerTests.java

@@ -53,7 +53,7 @@ public class SpringFunctionInitializerTests {
 	public void functionBean() {
 		initializer = new SpringFunctionInitializer(FluxFunctionConfig.class);
 		initializer.initialize();
-		Flux<?> result = initializer.apply(Flux.just(new Foo()));
+		Flux<?> result = Flux.from(initializer.apply(Flux.just(new Foo())));
 		assertThat(result.blockFirst()).isInstanceOf(Bar.class);
 	}
 
@@ -61,7 +61,7 @@ public class SpringFunctionInitializerTests {
 	public void functionCatalog() {
 		initializer = new SpringFunctionInitializer(FunctionConfig.class);
 		initializer.initialize();
-		Flux<?> result = initializer.apply(Flux.just(new Foo()));
+		Flux<?> result = Flux.from(initializer.apply(Flux.just(new Foo())));
 		assertThat(result.blockFirst()).isInstanceOf(Bar.class);
 	}
 
@@ -70,7 +70,7 @@ public class SpringFunctionInitializerTests {
 		initializer = new SpringFunctionInitializer(NamedFunctionConfig.class);
 		System.setProperty("function.name", "other");
 		initializer.initialize();
-		Flux<?> result = initializer.apply(Flux.just(new Foo()));
+		Flux<?> result = Flux.from(initializer.apply(Flux.just(new Foo())));
 		assertThat(result.blockFirst()).isInstanceOf(Bar.class);
 	}
 
@@ -78,7 +78,7 @@ public class SpringFunctionInitializerTests {
 	public void consumerCatalog() {
 		initializer = new SpringFunctionInitializer(ConsumerConfig.class);
 		initializer.initialize();
-		Flux<?> result = initializer.apply(Flux.just(new Foo()));
+		Flux<?> result = Flux.from(initializer.apply(Flux.just(new Foo())));
 		assertThat(result.toStream().collect(Collectors.toList())).isEmpty();
 	}
 
@@ -86,7 +86,7 @@ public class SpringFunctionInitializerTests {
 	public void supplierCatalog() {
 		initializer = new SpringFunctionInitializer(SupplierConfig.class);
 		initializer.initialize();
-		Flux<?> result = initializer.apply(Flux.empty());
+		Flux<?> result = Flux.from(initializer.apply(Flux.empty()));
 		assertThat(result.blockFirst()).isInstanceOf(Bar.class);
 	}