Restore customization of PropertyResolver

This commit reintroduces the ability to customize the PropertyResolver
to use in PropertySourcesPropertyResolver

See gh-26761

spring-context/src/main/java/org/springframework/context/support/PropertySourcesPlaceholderConfigurer.java

@@ -170,10 +170,17 @@ public String getProperty(String key) {
 			}
 		}
 
-		processProperties(beanFactory, new PropertySourcesPropertyResolver(this.propertySources));
+		processProperties(beanFactory, getPropertyResolver(this.propertySources));
 		this.appliedPropertySources = this.propertySources;
 	}
 
+	/**
+	 * Construct and provide a PropertyResolver from the given properties.
+	 */
+	public ConfigurablePropertyResolver getPropertyResolver(MutablePropertySources propertySources){
+		return new PropertySourcesPropertyResolver(propertySources);
+	}
+
 	/**
 	 * Visit each bean definition in the given bean factory and attempt to replace ${...} property
 	 * placeholders with values from the given properties.