Include multiple reports during site-generation.

To differentiate between the coverage of unit tests and integration tests, include a second execution definition for the failsafe plugin. This will be later reused during site generation, the name of the anchor is set with the new property `reportName`, it is crucial to:
- use maven-site-plugin version 3.3+
- define the `dataFile` as it's base name (`File.getName()`) is used as directory name in the site.
- define multiple report sets in the reporting section, see `jacoco-maven-plugin.test/it/it-site-failsafe/pom.xml` for an example how to do this.