Add more metadata to extension

[iawia002]

~1. Add a default language option~

• 
  2. Add static file directory configuration, requiring all static files to be stored in this directory
  3. Add extension provider configuration

ref https://github.com/kubesphere/issues/issues/1024 https://github.com/kubesphere/issues/issues/1097

[iawia002]

/cc @wansir @stoneshi-yunify

[stoneshi-yunify]

you may also provide a Validate func, to verify if a given file is a extension package, like the helm validate command.

[iawia002]

you may also provide a Validate func, to verify if a given file is a extension package, like the helm validate command.

I have provided this function as well, the validation steps will be gradually improved.

/cc @wansir @stoneshi-yunify

[wansir]

LGTM