Simplify generation of completion script

The idea is:

• Un-comment the current dummy completions
• Replace them with two actual completion implementations (one for connector list, one for task list), which return the corresponding kcctl command (e.g. "kcctl", "get-connectors-list")

Then, when running the generate command as described in the README, only one slight manual adjustment will be needed for replacing the quotes from the command with backticks, so to make this an actual command invocation.

Subsquently, there should be a command in kcctl for completion script generation (as described in the PicoCLI docs) instead of checking in the script into the kcctl repo. This command implementation ideally will take care of that backtick adjustment above.

[gunnarmorling]

With kcctl being installable via SDKMan soon, we should now add that ability to create the completion script via kcctl create-completion. Otherwise, users won’t have an easy way for obtaining the completion script.

[gunnarmorling]

I’ve pushed a commit for the first step described above, handling completions was just getting too unwieldy. What remains to be done is making completion generation an actual command of kcctl itself.