* kserve: Add distinct file for cluster-roles

When we install both KFServing 0.6.1 and KServe 0.7.0 we are using the
standalone version of KServe 0.7.0. But, the standalone KServe
installation does not include the aggregated roles for the KF
kubeflow-{admin,editor,viewer} roles.

This commit adds the needed clusterroles/bingings as a distinct file to
explicitly include them, when installing both KFServing and standalone
KServe.

Signed-off-by: Kimonas Sotirchos <kimwnasptd@arrikto.com>

* kserve: Use kubeflow overlay of MWA

In order to use the KServe MWA by default we will need to use the
kubeflow overlay. This overlay updates the VirtualService to use the
kubeflow/kubeflow-gateway Gateway, alongside the other KF apps.

Signed-off-by: Kimonas Sotirchos <kimwnasptd@arrikto.com>