Include an HTTP client in spring-boot-starter-restclient

[wilkinsona]

spring-boot-starter-webclient includes Reactor Netty as the preferred HTTP client. spring-boot-starter-restclient does not include an HTTP client dependency so the client will be based on the JDK's HTTP client. That may not be what we want. If it isn't, we should add our preferred HTTP client (Apache's?) to the starter.

[wilkinsona]

We were tempted to include the Apache HTTP components client to align with the detect method, however we have some concerns over its size. The JDK's client will (almost) always be available for no extra "cost". Adding the Apache HTTP components client will add 1.8MB+ to an application.

@bclozel also pointed out that the reactive side is a little different. There's benefit in terms of resource sharing to using Reactor Netty for both the client and the server. There's no such benefit with the blocking clients.