Spring Cloud Openfeign refactor load balance client so that users can add their custom implementation

Is your feature request related to a problem? Please describe.

My application needs to add a custom Client like FeignBlockingLoadBalancerClient, e.g.

``` public class CustomFeignBlockingLoadBalancerClient implements Client { ... }

`CustomFeignBlockingLoadBalancerClient`  will add some extensions to `FeignBlockingLoadBalancerClient`, like tracing. 

This works pretty well when using load-balanced clients. But when mixed with load-balanced and non-load-balanced clients, `FeignClientFactoryBean` hard coded to work with `FeignBlockingLoadBalancerClient` to get the delegate

    if (client != null) {
        if (client instanceof FeignBlockingLoadBalancerClient) {
            // not load balancing because we have a url,
            // but Spring Cloud LoadBalancer is on the classpath, so unwrap
            client = ((FeignBlockingLoadBalancerClient) client).getDelegate();
        }
        if (client instanceof RetryableFeignBlockingLoadBalancerClient) {
            // not load balancing because we have a url,
            // but Spring Cloud LoadBalancer is on the classpath, so unwrap
            client = ((RetryableFeignBlockingLoadBalancerClient) client).getDelegate();
        }
        builder.client(client);
    }

```

see https://github.com/spring-cloud/spring-cloud-openfeign/pull/776

Describe the solution you'd like see https://github.com/spring-cloud/spring-cloud-openfeign/pull/776

Describe alternatives you've considered see https://github.com/spring-cloud/spring-cloud-openfeign/pull/776

Additional context NA

This issue is created for future enhancement to loadbalancer client customization.

Comment From: OlgaMaciaszek

Hello, @liubao68 - we actually provide tracing solutions. Have you checked that out? In any case, I think it'd be ok to allow for custom FeignLoadBalancerClient solutions. This is not going to be a team priority, but feel free to submit a PR similar to the one you've created before, but against the main branch instead. Please make sure to add tests.