Expose builder for apache http5

[gmellemstrand]

This relates to #1443

In additon to the build method, I've added a new public method which adds the http tracing to the builder. This new method can be called instead of the build method in the builder is required in order to add additional options before building. The functionality should be identical for current users.

[codefromthecrypt]

@reta @kojilin can you consider this when updating httpclient5 for the 5.4 break?

[reta]

@gmellemstrand I believe, adding additional method like this would introduce conflicting (addTracingToBuilder vs build) instrumentation and confusion (when addTracingToBuilder should be used and why). I would recommend to have a factory method instead, fe:

Suggested change

	public HttpClientBuilder addTracingToBuilder(HttpClientBuilder builder) {
	public static HttpClient5CustomTracing newBuilder(HttpTracing httpTracing, HttpClientBuilder builder) {
	return new HttpClient5CustomTracing(httpTracing, builder);
	}

In this case the HttpClient5CustomTracing type would take care of adding tracing to the provided HttpClientBuilder as well as having a build() method with no args.

@codefromthecrypt does it make sense to you?