Add CRDs to flux check command

[stefanprodan]

Changes to flux check:

• Verify that the Flux CRDs are correctly registered on the cluster and print their version
• Error out if no controllers or crds are found

Example output:

$ flux check
► checking prerequisites
✔ Kubernetes 1.23.5-gke.1501 >=1.20.6-0
► checking controllers
✔ helm-controller: deployment ready
► ghcr.io/fluxcd/helm-controller:v0.22.1
✔ image-automation-controller: deployment ready
► ghcr.io/fluxcd/image-automation-controller:v0.23.2
✔ image-reflector-controller: deployment ready
► ghcr.io/fluxcd/image-reflector-controller:v0.19.1
✔ kustomize-controller: deployment ready
► ghcr.io/fluxcd/kustomize-controller:v0.26.1
✔ notification-controller: deployment ready
► ghcr.io/fluxcd/notification-controller:v0.24.0
✔ source-controller: deployment ready
► ghcr.io/fluxcd/source-controller:v0.25.5
► checking crds
✔ alerts.notification.toolkit.fluxcd.io/v1beta1
✔ buckets.source.toolkit.fluxcd.io/v1beta2
✔ gitrepositories.source.toolkit.fluxcd.io/v1beta2
✔ helmcharts.source.toolkit.fluxcd.io/v1beta2
✔ helmreleases.helm.toolkit.fluxcd.io/v2beta1
✔ helmrepositories.source.toolkit.fluxcd.io/v1beta2
✔ imagepolicies.image.toolkit.fluxcd.io/v1beta1
✔ imagerepositories.image.toolkit.fluxcd.io/v1beta1
✔ imageupdateautomations.image.toolkit.fluxcd.io/v1beta1
✔ kustomizations.kustomize.toolkit.fluxcd.io/v1beta2
✔ providers.notification.toolkit.fluxcd.io/v1beta1
✔ receivers.notification.toolkit.fluxcd.io/v1beta1
✔ all checks passed

Failed check example:

$ flux check -n test
► checking prerequisites
✔ Kubernetes 1.23.5-gke.1501 >=1.20.6-0
► checking controllers
✗ no controllers found in the 'test' namespace with the label selector 'app.kubernetes.io/part-of=flux'
► checking crds
✔ alerts.notification.toolkit.fluxcd.io/v1beta1
✔ buckets.source.toolkit.fluxcd.io/v1beta2
✔ gitrepositories.source.toolkit.fluxcd.io/v1beta2
✔ helmcharts.source.toolkit.fluxcd.io/v1beta2
✔ helmreleases.helm.toolkit.fluxcd.io/v2beta1
✔ helmrepositories.source.toolkit.fluxcd.io/v1beta2
✔ imagepolicies.image.toolkit.fluxcd.io/v1beta1
✔ imagerepositories.image.toolkit.fluxcd.io/v1beta1
✔ imageupdateautomations.image.toolkit.fluxcd.io/v1beta1
✔ kustomizations.kustomize.toolkit.fluxcd.io/v1beta2
✔ providers.notification.toolkit.fluxcd.io/v1beta1
✔ receivers.notification.toolkit.fluxcd.io/v1beta1
✗ check failed