feat: extend the existingJavaTypes to support use of existing enumerations.

[k-wall]

Description

Fixes #7045

Extends the existing existingJavaTypes of the java-generator-maven-plugin' to allow the generated code to refer to existing enumerations.

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] Feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change
• [ ] Chore (non-breaking change which doesn't affect codebase; test, version modification, documentation, etc.)

Checklist

• [x] Code contributed by me aligns with current project license: Apache 2.0
• [x] I Added CHANGELOG entry regarding this change
• [x] I have implemented unit tests to cover my changes
• [ ] I have added/updated the javadocs and other documentation accordingly
• [ ] No new bugs, code smells, etc. in SonarCloud report
• [x] I tested my code in Kubernetes
• [ ] I tested my code in OpenShift