[ClusterClass Operations] Add rebase controller

[salasberryfin]

This is issue is part of the epic #387.

Description

Add new controller to manage rebase operations: how changes are applied to downstream clusters created using ClusterClass.

This proposal contains detailed information on what the specific of this operation are.

Features

The rebase operation makes use of Fleet's functionality for matching and applying changes to downstream cluster and Turtles will add ClusterClass integration on top of it (and adapt specifications where we see fit) which will be based on a number of new features that must be implemented together with the controller:

• New ClusterUpgradeGroup type. The following sample definition can be used as base template.

// ClusterUpgradeGroupSpec defines the desired state of ClusterUpgradeGroup
type ClusterUpgradeGroupSpec struct {
	// +required
	ClassName string `json:"className"`

	// RolloutStrategy controls the rollout of bundles, by defining
	// partitions, canaries and percentages for cluster availability.
	// +optional
	RolloutStrategy *fleetv1.RolloutStrategy `json:"rolloutStrategy,omitempty"`

	// Targets refer to the clusters that should be upgraded.
	Targets []ClusterTargets `json:"targets,omitempty"`
}

• Define ClusterTargets including selectors for controlling which objects will be affected by the rebase operation.
• Prevent rebase conflicts where more than one ClusterUpgradeGroup include the same Cluster as ClusterTarget via validating webhooks.