Consider japicmp for testing backwards-compatibility

japicmp is what Flink uses to test backwards-compatibility, configuration here: https://github.com/apache/flink/blob/41d5875bfc272f2cd5c7e8c8523036684865c1ce/pom.xml#L1184

At our first stable release, we should activate this plugin too! (configuration to be determined by community discussion). We should have it installed and configured beforehand, informational-only, so that we have a sense of it.

Imported from Jira [BEAM-1131](https://issues.apache.org/jira/browse/BEAM-1131). Original Jira may contain additional context.
Reported by: kenn.

Start with the project's pom.xml and compare the referenced Apache Flink japicmp configuration. Review the original Jira context and community discussion to determine the compatibility scope and configuration. Done means japicmp is installed and configured for informational-only backwards-compatibility checks before the first stable release.

Written by the indexing model from the issue text.