Add support for running benchmarks with WebClient

[AbhyudayaSharma]

Adds support for running benchmarks using WebClient.

@jenkinsci/gsoc2019-role-strategy

[oleg-nenashev]

Would be need to do some refactoring to avoid need in inheriting JenkinsRule and de-facto having two wrappers inside every performance test. How closely is WebClient tied to JenkinsRule, would we be able to make it a static class from it (Maybe a new class name so that we retain binary compatibility)

[AbhyudayaSharma]

How closely is WebClient tied to JenkinsRule

From a quick look at the code, it is possible to extract WebClient from JenkinsRule. WebClient basically depends on the reference to the the JUnit test Description, Jenkins instance, it's URL and a private method related to CrumbIssuer. @oleg-nenashev Do you recommend to extract it out of JenkinsRule? Also, in the above test benchmark, we directly instantiate the WebClient just because it is running on a single thread. In role strategy, when we were running it on two threads, we had to create a separate state because it is not thread-safe.

https://github.com/jenkinsci/role-strategy-plugin/blob/master/src/test/java/jmh/benchmarks/WebClientBenchmark.java#L21-L29

From our benchmarks, the only thing I see that is different is the URL because the port it runs is different every time. Please let me know what you think about it.

[AbhyudayaSharma]

Or we could just have another utility class that creates a WebClient for each thread in the benchmark and the overall benchmark class could look something like:

@JmhBenchmark
public class SomeBenchmark {
    public static class State1 extends JmhJenkinsRule {}
    
    public static class ThreadState extends WebClientThreadState {}
    
    @Benchmark
    public void benchmark(State1 state1, ThreadState state2) {
        state2.webClient.goTo("");
    }
}

[oleg-nenashev]

Let's park it for a while I will release #141 without it, so we won't need to rush this PR. Also, a merge conflict