Improved reactive type descriptor support

ReactiveTypeDescriptor is now a top-level type with named constructors.
It also contains and provides a getter for the actual type.

Issue: SPR-14902

spring-core/src/test/java/org/springframework/core/convert/support/ReactiveAdapterRegistryTests.java

@@ -43,11 +43,13 @@ public class ReactiveAdapterRegistryTests {
 
 	private ReactiveAdapterRegistry adapterRegistry;
 
+
 	@Before
 	public void setUp() throws Exception {
 		this.adapterRegistry = new ReactiveAdapterRegistry();
 	}
 
+
 	@Test
 	public void getDefaultAdapters() throws Exception {
 		testMonoAdapter(Mono.class);