Add compatibility check to build pipeline to check plugin compatibility with example repository

[mkobit]

Example repository at https://github.com/mkobit/jenkins-pipeline-shared-library-example

• Use composite builds
• Deterministic revision of https://github.com/mkobit/jenkins-pipeline-shared-library-example
• Try to make use of shallow clone as much as possible
• Think about implications of making breaking changes in this plugin that would break example

[mkobit]

Going to see if this is doable as part of next set of tasks I tackle.