Introduce builder customizer for Lettuce `ClientResources`

[mp911de]

Customizing an existing ClientResources instance is difficult for integration libraries as it requires a BeanPostProcessor calling ClientResources.mutate(). Using BeanPostProcessor limits the ability of what Spring Beans can be injected during customization.

This came up while trying to set Lettuce's ClientResources.tracing(…) using a Spring Cloud Sleuth bean. It would be good to have a ClientResourcesBuilderCustomizer API to let Spring Boot create DefaultClientResources while being able to hook into builder customization.

[marcingrzejszczak]

This is the related commit in Spring Cloud Sleuth spring-cloud/spring-cloud-sleuth@6bca070