springboot Auto-configure HTTP client builders as lazy

Spring Boot is auto-configuring, if the required classes are present, builders for HTTP clients: RestTemplateBuilder for Spring MVC and WebClient.Builder for Spring WebFlux.

Many web applications only use one of them, or just don't use any. Creating such components does not take a lot of resources, but the underlying infrastructure (including codecs) can take startup time without doing anything useful for the application.

We could make those components lazy by default; such components are used by the application configuration to create new components at startup time, so making those builders lazy will not change the behavior here.