Necessary crumb support after Jenkins 2.176.2+

As #429 mentioned, Jenkins LTS after 2.176.2+ improved CSRF protection and make current release of this library not work. However, there is still not a PR to fix this issue. To take advantage of this excellent jenkins client api library hosted by jenkins organization, this may be the first priority issue to solve.

Another jenkins client api library mentioned on Jenkins official page [jenkins-rest](https://github.com/cdancy/jenkins-rest) has already merged a PR to fix this issue (see https://github.com/cdancy/jenkins-rest/pull/70), and it works for Jenkins LTS after 2.176.2+ after that.

@khmarbaise
If you think this is a good proposal, may I try to work on it based on the same approach and send a PR to this project? Thanks.

The issue names no source files or tests. Start by comparing this client's CSRF request handling with the approach in jenkins-rest pull request #70, then verify compatibility with Jenkins LTS versions after 2.176.2; done means requests from this Java API client work with the newer crumb requirements.

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