Support fish on eksctl completion

[gjmveloso]

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Why do you want this feature?
fish is an interactive shell that is gaining popularity as it's easier to use than bash or even zsh. Command-line applications that pushes man pages got completions generated by default but this is not the case of eksctl

What feature/behavior/change do you want?
Having an option to generate fish completion by doing eksctl completion fish

Do not hesitate, when appropriate, to share the exact commands or API you would like, and/or to share a diagram (e.g.: asciiflow.com): "a picture is worth a thousand words".

[af12066]

ref. spf13/cobra#754

[sayboras]

ref. spf13/cobra#1048