Support dynamic URI building for the upstream service

Is your feature request related to a problem? Please describe.
I would like to be able to build upstream service URI based on components of the request. For instance given a request with the path /api/{serviceName} I would like to build the upstream URI using the serviceName piece like so:

Map ports = Map.of("service1", 8080, "service2", 8081);
URI upstreamUri = "http://host:" + ports.get(serviceName) + "/";

Describe the solution you'd like
For the fluent routes (RouteLocatorBuilder) one solution might be to introduce another method e.g. uriBuilder which will have access to request details:

@Bean
public RouteLocator routes(RouteLocatorBuilder builder) {
  return builder.routes()
    .route("dynamic-route", predicate -> predicate.path(/api/**))
    .uriBuilder(request -> {
      String serviceName = "todo"; //extract service name from the reqeust's path
      return "http://host:" + ports.get(serviceName) + "/"
    })
    .build();
}

Describe alternatives you've considered
I couldn't find any alternatives. The closest I could find was the RewritePath GatewayFilter but it allows only for rewriting the path part of the URI.

Start by reviewing the fluent RouteLocatorBuilder API and the RewritePath GatewayFilter, which the issue identifies as the closest existing entry points. Determine how request details could reach upstream URI construction and define done as supporting request-dependent upstream URI components while preserving existing route behavior.