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-r2dbc/src/main/java/org/springframework/r2dbc/connection/lookup/AbstractRoutingConnectionFactory.java

@@ -127,6 +127,14 @@ public abstract class AbstractRoutingConnectionFactory implements ConnectionFact
 
 	@Override
 	public void afterPropertiesSet() {
+		initialize();
+	}
+
+	/**
+	 * Synchronizes targetConnectionFactories to resolvedConnectionFactories
+	 * and defaultTargetConnectionFactory to resolvedDefaultConnectionFactory.
+	 */
+	public void initialize() {
 		Assert.notNull(this.targetConnectionFactories, "Property 'targetConnectionFactories' must not be null");
 
 		this.resolvedConnectionFactories = CollectionUtils.newHashMap(this.targetConnectionFactories.size());