If triggered build is from FlowRun type, then iterate through its jobs graph adding all downstream jobs build urls to the message that will be posted to github pull request.

Please review my changes, and consider changing what you think it's better for the plugin. I'm not sure if I could have achieved this in a easier/extensible way, so please send me your feedback about how to do it. Thanks in advance
Yes, there is a getPlugin method:
```java
Plugin buildFlowPlugin = Jenkins.getInstance().getPlugin("build-flow-plugin");
```
which will couple this build URL generation to the BuildFlow only.

But, what will it happen if we want to plug in a new build type? If we keep the instanceof type (or something architecturally better), we could add new buildUrl types, depending of the build type, only implementing the url generation on a manager.

```java
if (build instanceof FlowRun) {
    return new BuildFlowBuildUrlManager(build);
}
else if (build instanceof AnotherBuildType) {
    return new AnotherBuildTypeBuildUrlManager(build);
}
```