Add a page object for the snippet generator

[uhafner]

I use this PO in the ATH tests of the warnings plugin:
https://github.com/jenkinsci/warnings-ng-plugin/blob/5effdb04eaa5dc1025085ac41257e5a6ade4de80/ui-tests/src/test/java/io/jenkins/plugins/analysis/warnings/SnippetGeneratorUiTest.java

[jtnord]

please avoid introducing new POs with elastic sleep where they can be avoided.

if the callers are never going to call generate script more than once you can wait wait.until the textarea is not empty. (and add this fact to the javadoc)
if it is going to call it more than once with different options then it should be possible to get a before and wait until it is not the same.