JENKINS-56553 Make proxy config compatible with JCasC without workarounds on...

JENKINS-56553 Make proxy config compatible with JCasC without workarounds on the plugin side (#3935)

* JENKINS-56553 Make proxy config compatible with JCASC

* Use DataBoundSetters as other fields are optional

* Fix Jelly for ProxyConfiguration

* Use default field name now that field matches

* Fix validate proxy

* Change field type

* After review

* Update core/src/main/java/jenkins/model/Jenkins.java

Co-Authored-By: timja <t.jacomb@kainos.com>

* Update core/src/main/java/jenkins/model/Jenkins.java

Co-Authored-By: timja <t.jacomb@kainos.com>

* Update core/src/main/java/hudson/ProxyConfiguration.java

Co-Authored-By: timja <t.jacomb@kainos.com>

* Update core/src/main/java/jenkins/model/Jenkins.java

Co-Authored-By: timja <t.jacomb@kainos.com>

* Update core/src/main/java/jenkins/model/Jenkins.java

Co-Authored-By: timja <t.jacomb@kainos.com>

* Update core/src/main/java/hudson/ProxyConfiguration.java

Co-Authored-By: Oleg Nenashev <o.v.nenashev@gmail.com>

* Tweak javadoc

* Align fields

* Update core/src/main/java/hudson/ProxyConfiguration.java

Co-Authored-By: Adrien Lecharpentier <adrien.lecharpentier@gmail.com>