Improve developer experience: improved autocomplete with context aware options

[Richard87]

For example in rx get application --application f we could list available applications the user have access to

func init() {
	getCmd.AddCommand(getApplicationCmd)
	getApplicationCmd.Flags().StringP(flagnames.Application, "a", "", "Name of the application")
	getApplicationCmd.RegisterFlagCompletionFunc(flagnames.Application, func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
		return []string{"foo", "bar", "baz"}, cobra.ShellCompDirectiveNoFileComp
	})
	setContextSpecificPersistentFlags(getApplicationCmd)
}

Just listing applications, environments, components, jobs, secrets and variables would make rx way nicer to use :)

DoD
Implement autocomplete for values for the following 6 properties:

• applications
• environments
• components
• secrets
• variables
• dns alias
• jobs