fix: add finalizer to credentials reference

[cbzzz]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
Adds a unique finalizer to the credentials reference Secret (if .spec.CredentialsRef is set) when creating Linode{Cluster,Machine,VPC} resources and removes it upon deletion. The finalizer is unique in case you have a shared Secret (e.g. KIND.GROUP/NAMESPACE.NAME).

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:
This allows users to use kubectl delete with the output of clusterctl generate, e.g.:

clusterctl generate cluster $CLUSTER_NAME \
  --kubernetes-version v1.29.1 \
  --infrastructure akamai-linode:v0.0.0 \
  --flavor clusterclass-kubeadm \
  | kubectl delete -f -

TODOs:

• squashed commits
• includes documentation
• adds unit tests
• adds or updates e2e tests

Testing:

1. Create a cluster:

$ clusterctl generate cluster $CLUSTER_NAME \
  --kubernetes-version v1.29.1 \
  --infrastructure linode:0.0.0 \
  | kubectl apply -f -

2. Check the cluster credentials Secret for an object finalizer:

$ kubectl get secret ${CLUSTER_NAME}-credentials -o yaml | yq .metadata.finalizers
- linodecluster.infrastructure.cluster.x-k8s.io/default.${CLUSTER_NAME}

3. Delete the cluster:

$ clusterctl generate cluster $CLUSTER_NAME \
  --kubernetes-version v1.29.1 \
  --infrastructure linode:0.0.0 \
  | kubectl delete -f -

4. All cluster resources should clean up successfully:

$ kubectl get cluster-api,secrets
No resources found in default namespace.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 24 lines in your changes are missing coverage. Please review.

Project coverage is 64.87%. Comparing base (2f81203) to head (286ac18).

Files	Patch %	Lines
controller/linodecluster_controller.go	0.00%	6 Missing and 3 partials ⚠️
controller/linodemachine_controller.go	0.00%	8 Missing and 1 partial ⚠️
controller/linodevpc_controller.go	0.00%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #288      +/-   ##
==========================================
+ Coverage   64.40%   64.87%   +0.47%     
==========================================
  Files          30       30              
  Lines        1750     1842      +92     
==========================================
+ Hits         1127     1195      +68     
- Misses        574      592      +18     
- Partials       49       55       +6     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[eljohnson92]

This change looks good to me overall, can we just add some unit tests for the new functionality where unit test frameworks are already in place?

[AshleyDumaine]

This might be out of scope for this PR, but we might want to prefix some extra resources with the cluster name like the common-init-files Secret and the helmchartproxies to support the k delete -f workflow in the case there are multiple workload clusters in one namespace:

clusterctl generate cluster $CLUSTER_NAME \
  --kubernetes-version v1.29.1 \
  --infrastructure linode:v0.0.0 \
  | kubectl delete -f -
secret "cleanup-credentials" deleted
secret "common-init-files" deleted
secret "linode-cleanup-crs-0" deleted
helmchartproxy.addons.cluster.x-k8s.io "cilium" deleted
helmchartproxy.addons.cluster.x-k8s.io "cilium-ipv6" deleted
helmchartproxy.addons.cluster.x-k8s.io "csi-driver-linode" deleted
helmchartproxy.addons.cluster.x-k8s.io "linode-cloud-controller-manager" deleted
clusterresourceset.addons.cluster.x-k8s.io "cleanup-crs-0" deleted
kubeadmconfigtemplate.bootstrap.cluster.x-k8s.io "cleanup-md-0" deleted
cluster.cluster.x-k8s.io "cleanup" deleted
machinedeployment.cluster.x-k8s.io "cleanup-md-0" deleted
kubeadmcontrolplane.controlplane.cluster.x-k8s.io "cleanup-control-plane" deleted
linodecluster.infrastructure.cluster.x-k8s.io "cleanup" deleted
linodemachinetemplate.infrastructure.cluster.x-k8s.io "cleanup-control-plane" deleted
linodemachinetemplate.infrastructure.cluster.x-k8s.io "cleanup-md-0" deleted
linodevpc.infrastructure.cluster.x-k8s.io "cleanup" deleted

[eljohnson92]

agreed we should scope addons to the cluster they are associated with, I think that is out of scope for this PR but should be addressed in #289 for CCM/Cilium and we can create a separate follow up for the remaining addons