[CLI] Add proxy authentication support to jenkins-cli.jar when it connects to a Jenkins master

[CLI] Add proxy authentication support to jenkins-cli.jar when it connects to a Jenkins master.

Sample command line:

```
java \
   -Dhttp.proxyHost=proxy.example.com -Dhttp.proxyPort=3128 -Dhttp.proxyUser=myuser -Dhttp.proxyPassword=mypassword \
   -Dhttps.proxyHost=proxy.example.com -Dhttps.proxyPort=3128 -Dhttps.proxyUser=myuser -Dhttps.proxyPassword=mypassword \
   -jar jenkins-cli.jar -s https://jenkins.example.com ...
```

Don't forget to declare both http and https configs (`-Dhttp.proxyHost` and `-Dhttps.proxyHost` ...) :
* `-Dhttps.proxyHost` is needed because the Jenkins URL (`-s https://jenkins.example.com`) is using https
* `-Dhttp.proxyHost`because internally, the proxy selector presents to the Authenticator the invocation to the proxy as an http (not https) call