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

[kwin]

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