Factor-out CI template for maven-based projects

Description

We (~"team::HR Systems") have currently got at least two projects which contain very similar CI configuration to build, test and publish maven-based Java projects. This CI template has been a lift-and-light-edit from the GitLab's AutoDevops implementation, we should bring them together and implement a single CI template that can be used by all. The project are:

• Java Common - the most standard of all, implementing two jobs for mvn verify and mvn deploy
• WRS - a little more complex as private packages need to be fetched which requires a GitLab token to be fetched from Secrets Manager, additionally this project doesn't publish a jar as the application gets packaged into a Docker image outside this CI template.

Acceptance criteria

• A maven CI template is defined in this repo which meets the needs of existing maven-based projects and caters for the build, test and publish jobs currently defined within the project-level CI template,
• The projects mentioned above are migrated over to use the shared template,