Introduce initialize() in AbstractRouting[DataSource|ConnectionFactory]

This commit introduces initialize() methods in
AbstractRoutingDataSource and AbstractRoutingConnectionFactory as an
alternative to invoking afterPropertiesSet().

Closes gh-31248

spring-jdbc/src/main/java/org/springframework/jdbc/datasource/lookup/AbstractRoutingDataSource.java

@@ -116,6 +116,15 @@ public abstract class AbstractRoutingDataSource extends AbstractDataSource imple
 
 	@Override
 	public void afterPropertiesSet() {
+		initialize();
+	}
+
+	/**
+	 * Synchronizes targetDataSources to resolvedDataSources
+	 * and defaultTargetDataSource to resolvedDefaultDataSource.
+	 * @throws IllegalArgumentException in case of targetDataSources is null
+	 */
+	public void initialize() {
 		if (this.targetDataSources == null) {
 			throw new IllegalArgumentException("Property 'targetDataSources' is required");
 		}