What this PR does.

• Grouped all event classes inside their own package events. They all extends an abstract class called AbstractDatadogEvent with common methods to all event implementations. I all added an common interface called DatadogClient.
• I also created a model called BuildData to collect build informations. Before we would use a JSONObject that was harder to work with.
• I changed the DatadogHttpRequest class into the DatadogHttpClient. This class cannot be instantiated and can now be leased from the descriptor. The same way we used to do with the StatdDClient. DatadogHttpClient implement a new interface called DatadogClient. This interface is used to create stubs for testings. See more details below.
• DatadogHttpClient now only has methods related to submitting data to datadog. As a consequence, I was able to remove "duplicated-ish" submission methods (eg: the gauge or the serviceCheck method) that were part of other classes like the DatadogBuildListener. I was also able to group all the submission logic inside this new client (that includes constants variables as well)
• Code for the onStarted, the onCompleted, the onCheckout and the doRun method is now simpler and more readable.
• This code refactoring also simplified the content of the DatadogUtilities class. I moved methods to better locations if needed. When used only once, I often removed the method. When called only within the same class, I changed their scope to private.
• I added test for all events classes, I created a DatadogClientStub and a DatadogStatsDClientStub classes to extend testing capabilities. As a consequence I created more tests to the DatadogBuildListener (onStarted method in particular)
• General linting

What is next

• Add more tests in general over time.