Sign clusterctl binaries with cosign

### What would you like to be added (User Story)?

We are already signing our images with cosign, and this issue tracks the signing of `clusterctl` binary with cosign.
This will be followed by documenting verifications steps which is being tracked in #9292

### Detailed Description

Sign `clusterctl` binary with cosign and all `*.sig` and `*.cert` file for verification.

Ref: https://kubernetes.io/docs/tasks/administer-cluster/verify-signed-artifacts/#verifying-binary-signatures

cc @nawazkh @furkatgofurov7 // for 1.6 release

### Anything else you would like to add?

_No response_

### Label(s) to be applied

/kind feature
One or more /area label. See https://github.com/kubernetes-sigs/cluster-api/labels?q=area for the list of labels.

Start by tracing how the clusterctl binary is built and released, then identify the existing image-signing workflow and its cosign usage. Use the linked Kubernetes verification guidance as a reference; done means clusterctl has accompanying *.sig and *.cert files that can be used to verify the binary.

Written by the indexing model from the issue text.