[java-generator] make the Maven plugin(and CLI?) able to fetch CRDs from URLs

[andreaTP]

Is your enhancement related to a problem? Please describe

Currently, when we need to download a CRD and generate the corresponding Java code we need to leverage some external Maven plugin, such as:
https://github.com/bf2fc6cc711aee1a0c2a/e2e-test-suite/pull/417/files#diff-9c5fb3d1b7e3b0f54bc5c4182965c4fe1f9023d449017cece3005d3f90e8e4d8R284-R338

This is tedious, verbose and error-prone; instead, the logic to actually fetch the resources might be simple enough for the most common use cases.

Describe the solution you'd like

The Maven plugin and the CLI should be able to fetch CRDs from URLs (possibly with a built-in caching as well).

Describe alternatives you've considered

No response

Additional context

No response