add Jenkins proxy configuration support

[moritzrupp]

To support larger corporations, the support for proxies is added.
The generic proxy configuration of Jenkins is used to communicate via proxy with the ServiceNow instance.

Referecne: #27

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@chiarnglin

[tdprot]

@chiarng @kamilgo @basil
Is there any reason that this pull request, which would be essential for enterprise environments, is not merged?

[basil]

I am just a Jenkins core maintainer, not a maintainer of this plugin: I only adopted this plugin temporarily to ship some changes adapting to a recent Jenkins core release, and I don't use ServiceNow. Since this plugin appears to be maintained by a corporate team at ServiceNow, I don't want to interfere with feature requests like this, so I will leave it up to them, but I can confirm this is a valid use case, and the implementation looks correct to me. See also jenkinsci/jenkins#7398 which attempts to introduce a high-level API for this use case in more recent versions of Jenkins core. (The testing notes in that PR might be interesting.) Assuming this PR has been properly tested, I would highly recommend the ServiceNow developers team merge and release this PR.