Add a "Plugin Registry API" to retrieve the list of plugins

[skabashnyuk]

Description

Since now metha.yaml are served by apache and we don't want to introduce some API server
we decided to create index.json file with a list of all existed plugins and editors.

[skabashnyuk]

@garagatyi @ashumilova @evidolob
are you ok with such content of index.json

[
  {
    "name": "che-dummy-plugin",
    "version": "0.0.1"
  },
  {
    "name": "che-editor-theia",
    "version": "1.0.0"
  },
  {
    "name": "che-service-plugin",
    "version": "0.0.1"
  }
]

[garagatyi]

I'm ok with this solution

[benoitf]

I would say that the type is missing. (Che plugin or che editor)

[garagatyi]

Yes, you are right. I created an issue for that #10781