Upgrading a cluster via Provisioner's API

[janmedrek]

Description

In the current state, it is possible only to upgrade Kyma installation on a provisioned Runtime. In some scenarios, there is also a requirement for upgrading the cluster itself - number of nodes, K8S version, machine type. We would like to have a possibility to realize this via the Provisioner.

Reasons

Access to this functionality via Provisioner's API will allow preserving a consistent data state (Gardener - DB) and enable automation of upgrade actions.

Acceptance Criteria

• New mutation introduced for upgrading (changing) cluster config
• Provisioner accepts the new ClusterConfigInput and applies it to the existing cluster
• Documentation updated