Implement HttpClient based Zipkin sender

[mhalbritter]

Currently, we have 3 Zipkin senders:

• URLConnectionSender, needs zipkin-sender-urlconnection
• RestTemplate based one, needs spring-web
• WebClient based one, needs spring-webflux

With the changes in #39049, the webclient one will likely be removed, as the Zipkin sender interface is no longer async by default and we have to call block.

We should implement a JDK HttpClient based sender, which can be used without any dependencies.

[mhalbritter]

Some WIP in https://github.com/mhalbritter/spring-boot/tree/mh/39545-implement-httpclient-based-zipkin-sender.

[wilkinsona]

I wonder if we should only provide an HttpClient-based sender. It would simplify things a little and give us less code to maintain if we got rid of the other two as well as the WebClient-based one.

[mhalbritter]

Yeah, I think that's a good idea. I'll try to get the HttpClient sender into 3.3.x and then we can deprecate the other ones.

[mhalbritter]

Blocked until #39049 is merged.