Allow java-generator-maven-plugin reuse an existing enum

[k-wall]

Is your enhancement related to a problem? Please describe

I am using CRD -> Java POJO.

In my API, I have the same enumeration appear in several places in the model (across several CRDs). Currently java-generator-maven-plugin creates an enumeration for each each instance, even though, the resulting code is the same. This makes for awkward code.

java-generator-maven-plugin's existingJavaTypes allows you to tell the plugin to reuse an existing Java class, but unfortunately, it does not support Java Enums.

Relates to: #7015

Describe the solution you'd like

An extension to the existingJavaTypes feature to support enums.

Describe alternatives you've considered

No response

Additional context

No response