Allow definition of WebClient.Builder in custom configuration referenced in @RetrofitClient or @EnableRetrofitClients

[michael-wirth]

Not sure if this is a bug or a missing feature. The WebClient.Builder can't be defined in a custom Configuration defined in a child applicationcontext for each RetrofitClient.

Describe the solution you'd like

• declare WebClient.Builder in a custom Configuration and lookup the bean in the child applicationcontext
• support bean aliases to use the same WebClient.Builder in multiple RetrofitClient

Describe alternatives you've considered none

[OlgaMaciaszek]

Hello @michael-wirth, this is a model we've had for any similar child-context- based implementations, such as SC OpenFeign and SC LoadBalancer and we've never had any requests for this, which, by itself, is not a reason to not to add this feature. However, there's a feature parity issue between the load-balanced and non-load-balanced version here. A @LoadBalanced WebClient.Builder could not be declared here, cause it would not be properly processed by [LoadBalancerWebClientBuilderPostProcessor](https://github.com/spring-cloud/spring-cloud-commons/blob/main/spring-cloud-commons/src/main/java/org/springframework/cloud/client/loadbalancer/reactive/LoadBalancerWebClientBuilderBeanPostProcessor.java) and clients would not resolved URLs from serviceIds. I don't think should add a feature that would only work on non-loadbalanced version.