Config.autoConfigure should support multiple files

[manusa]

Description

Part of #6516

As a final step before being able to complete #6240 and merge a reviewed and updated version of #6295, the method Config.autoConfigure(Config, String) needs improvement to support multiple kubeconfig files.

When a kubeconfig environment variable or System property is provided with multiple paths, the method should be able to process all of the configs in the provided paths.

The logic for merging is explained in #6551 and was already implemented in #6553.

Only additional wiring and tests are needed.