JENKINS-58753: Jenkins proxy settings support.

[jorgeyanesdiez]

These changes should fix the issue found here

I have added a new setting to afford the use of the proxy configuration in Jenkins (advanced tab of the plugins manager page, programmatically via hudson.ProxyConfiguration).

I refactored the existing io.jenkins.plugins.zoom.ZoomNotifyClient class to be able to reuse most of the code without any functional changes. In other words, if the Jenkins proxy is not configured at all, not configured correctly, or the new setting prevents the configuration from being used, the plugin behaves exactly as it did before.

Since proxy settings may be set at both via JVM args and via the Jenkins configuration, the new setting is useful to force the new behavior off.

The new proxy behaviour is also slightly less performant than the existing behavior because
(a) proxy settings are checked on every invocation
(b) a new httpClient is created for every proxied request.
Both are by design. This is so that a restart is not needed when proxy settings are modified, and also because the proxy configuration must be set via the httpclient builder.

If you feel this code is ok, please merge it for the next version. I have tested it internally and it is working great for me.

[jorgeyanesdiez]

Thanks for merging! :) Looking forward to the next version.

[zoomdevplatform]

The new version is online now, thanks so much for your contribution!