Add missing completion support for fish and PowerShell shells

[tuhtah]

Feature request

CLI completion support for the Bash and ZSH shell is provided using the underlying spf13/cobra library. Cobra additionally supports fish and PowerShell completion support out of the box, see: Generating shell completions

Use case

Other than providing the same discoverability and convenience for fish and PowerShell users it will bring the kn client in line with other plugins (e.g. quickstart) that have chosen to enable completion support for all 4 supported shells.