plugin feature

[tedteng]

What this PR does / why we need it:
introduce plugins feature

(N/A:N/A)➜  plugins ls -lrt
total 3840
-rw-r--r-- 1 xxxxx 3930736 May 12 12:56 example.so

go run main.go
...
Usage:
  gardenctl [command]

Available Commands:
  ....
  example      A brief description of your command 
 

go run main.go example
Test called

Which issue(s) this PR fixes:
Fixes #299

Special notes for your reviewer:

var NewCmd = &cobra.Command{  //must be `NewCmd` name

when "hidden:false", make gen-markdown will generate all documentation, including the plugin's description, because https://github.com/gardener/gardenctl-v2/compare/master...tedteng:gardenctl-v2:plugins?expand=1#diff-f36fcc143c1388c7c89b28b165cfb74d7f4c00fd6e58667a5594437f13180436R127-R129 and plugins(example.so) use cobra library as well.

Release note:

introduce plugins feature

[petersutter]

Hi @tedteng, are you aware of all the downsides that come with the golang plugins? If not I recommend checking out e.g. https://www.reddit.com/r/golang/comments/b6h8qq/is_anyone_actually_using_go_plugins/. Also (it's mentioned somewhere in the comments in this thread) windows is not supported.

[tedteng]

ah, didn't aware of it. my initial idea was to implement some less common features using plugins, if they proved useful, then migrate them into the main code if needed. the experiment in my local shows that the lower golang version main code cannot recognize higher golang version plugins and only works with higher golang version in main code with low/same version of plugins