Gerrit Event Trigger - Add Private & WIP State changed

[zeralight]

Since Gerrit 2.15, new events hooks have been introduced which can be used to trigger new Builds:

• Private state changed
• Work In Progress state changed
• Draft Publication removed

Ref: https://www.gerritcodereview.com/2.15.html#new-workflows

In this PR, we add support for the new events and keep the removed one for compatibility with Gerrit < 2.15.

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[basil]

The argument to the @since tag refers to the version of this repository, not the version of some other repository; 2.15 is not a valid version of Job DSL.

Why are the methods of GerritEventContext not kept sorted in lexicographical order?

Why is this PR not in draft state when TriggerContextSpec is consistently failing?

Speaking of tests, how was this PR tested?