Create ReactorNettyHttpClientMapper for configuring HttpClient

[bclozel]

Spring Framework provides a ReactorClientHttpConnector for WebClient, backed by Reactor Netty as an HTTP client. This connector implementation accepts a mapper Function<HttpClient, HttpClient> for customizing the HttpClient during its instantiation.

We should provide here a ReactorNettyHttpClientMapper interface that developers can use and declare as beans to configure HttpClient when used by the WebClient.Builder provided by Spring Boot.