Support fish completion for clusterctl

[kranurag7]

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

As of now, clusterctl doesn't support fish shell completion. I mostly use fish shell and would like to have completion for fish shell as well.

We use cobra under the hood and other cobra based CLIs, including kubectl support completion for fish shell.
Ref: https://pkg.go.dev/github.com/spf13/cobra#Command.GenFishCompletion

Detailed Description

$ clusterctl completion fish
Error: unsupported shell type "fish"

Anything else you would like to add?

I'll be happy to create a patch once it's triaged.

Label(s) to be applied

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

[fabriziopandini]

/triage accepted
/assign @kranurag7
/area clusterctl