Merge https://github.com/apache/maven-jenkins-env into this repository

This is just a downstream dependency from this library and consists out of two custom steps.:

1. https://github.com/apache/maven-jenkins-env/blob/master/vars/jenkinsEnv.groovy
2. https://github.com/apache/maven-jenkins-env/blob/master/vars/jenkinsNotify.groovy

So merging this into this library eases development/testing and reduces configuration complexity.

Compare with the Slack discussion at https://the-asf.slack.com/archives/C7Q9JB404/p1780991739792119

Start by reviewing vars/jenkinsEnv.groovy and vars/jenkinsNotify.groovy in the downstream maven-jenkins-env repository, then compare the Slack discussion for integration expectations. Done means both custom steps are incorporated into this library and its development, testing, and configuration no longer require the downstream dependency.

Written by the indexing model from the issue text.