Expose http builder for Apache http5 implementation

[gmellemstrand]

Feature

With the addition of the brave.httpclient5 package all the handling of the builder object is done internally, and the only option is to call the build method and get the finished client object. This happens in the HttpClient5Tracing class.

Rationale

There can be many cases, when integrating with other libraries, where you want to add other options to the http builder besides what is being added by Brave. This option was also available in the http4 implementation, but was not added to the http5 implementation. Adding a new public method for this is a minimal change.

Example Scenario

In our case we are in the process of upgrading dropwizard-zipkin to Dropwizard 3, which requires the use of Apache http5 (and in turn the Brave http5 implementation).