Add Bash and Zsh completion scripts (#22646)

This PR adds contrib scripts for bash and zsh completion.

Simply call:

```bash
source contrib/autocompletion/bash_autocomplete
```

or for Zsh:

```bash
source contrib/autocompletion/zsh_autocomplete
```

Signed-off-by: Andrew Thornton <art27@cantab.net>

---------

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: a1012112796 <1012112796@qq.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

main.go

@@ -113,6 +113,8 @@ arguments - which can alternatively be run by running the subcommand web.`
 		setFlagsAndBeforeOnSubcommands(&app.Commands[i], defaultFlags, establishCustomPath)
 	}
 
+	app.EnableBashCompletion = true
+
 	err := app.Run(os.Args)
 	if err != nil {
 		log.Fatal("Failed to run app with %s: %v", os.Args, err)