Build PRs from non-committers

As outlined in https://issues.apache.org/jira/browse/INFRA-18748?focusedCommentId=16885091&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-16885091 and discussed in https://lists.apache.org/thread.html/7ba9424ae1cadd61363a5c6e7d12dec9f5b424b1f68e27915032fbab@%3Cprivate.infra.apache.org%3E it is no longer allowed to automatically build PRs from non-committers on ASF infra.

Therefore another solution needs to be found to be able to validate PRs from contributors. This validation should include at least a Maven build and the validation of SonarQube rules.

Several options come to my mind

1. Use CloudBees Code Ship https://app.codeship.com/home. Currently it is unclear whether there is a dedicated ASF account. I asked about it in https://issues.apache.org/jira/browse/INFRA-18973. Preferred option as probably the Jenkinsfile can be reused.
2. Use Travis CI. There is a dedicated ASF account but it a) needs to be enabled by INFRA per project and b) requires to maintain another build script next to the Jenkinsfile we already have
3. Use another 3rd party build provider

This was also discussed during the Sling Hackathon 2019 (https://lists.apache.org/thread.html/fb675eda239779450943623490e5a232786464df1dfb1feac5ec4ee0@%3Cdev.sling.apache.org%3E).