Add an ability to use Devfile as part of Stack API

[sleshchenko]

Description

This is issues about adding an ability to use Defile as part of Stack.
It should cover Stacks for End-User user story of the following epic #12924.

It includes reworking Stack API in the same way as we did with Workspace API.
Add a new field devfile to stack object, modify REST API, DAOs to respect it.
Then stack can be defined with workspace config or devfile mutually exclusive.

As an example of Stack with devfile defined

Stacks with Devfile in JSON format

  {
    "id": "che7-preview",
    "creator": "ide",
    "name": "Che 7",
    "description": "Workspace.next sidecars and Theia as IDE",
    "scope": "general",
    "tags": [
      "ws.next",
      "Theia",
      "Java",
      "JDK",
      "Node.JS",
      "NPM",
      "Yeoman"
    ],
    "components": [
      {
        "name": "Centos",
        "version": "7"
      },
      {
        "name": "OpenJDK",
        "version": "1.8.0_181"
      },
      {
        "name": "NodeJS",
        "version": "8.12.0"
      },
      {
        "name": "NPM",
        "version": "6.4.1"
      }
    ],
    "devfile": {
      "name": "che7",
      "components": [
        {
          "type": "cheEditor",
          "id": "eclipse/che-theia/next"
        },
        {
          "type": "chePlugin",
          "id": "eclipse/che-machine-exec-plugin/0.0.1"
        },
        {
          "type": "kubernetes",
          "referenceContent": "kind: List\nitems:\n - \n  apiVersion: v1\n  kind: Pod\n  metadata:\n   name: ws\n  spec:\n   containers:\n    - \n     image: 'eclipse/che-dev:nightly'\n     name: dev\n     resources:\n      limits:\n       memory: 512Mi\n"
        }
      ]
    },
    "stackIcon": {
      "name": "type-che7.svg",
      "mediaType": "image/svg+xml"
    }
  }

Stacks with Devfile in YAML format

---
id: che7-preview
creator: ide
name: Che 7
description: Workspace.next sidecars and Theia as IDE
scope: general
tags:
- ws.next
- Theia
- Java
- JDK
- Node.JS
- NPM
- Yeoman
components:
- name: Centos
  version: '7'
- name: OpenJDK
  version: 1.8.0_181
- name: NodeJS
  version: 8.12.0
- name: NPM
  version: 6.4.1
devfile:
  name: che7
  components:
  - type: cheEditor
    id: eclipse/che-theia/next
  - type: chePlugin
    id: eclipse/che-machine-exec-plugin/0.0.1
  - type: kubernetes
    referenceContent: |
      kind: List
      items:
      - 
      apiVersion: v1
      kind: Pod
      metadata:
        name: ws
      spec:
        containers:
        - image: 'eclipse/che-dev:nightly'
          name: dev
          resources:
            limits:
             memory: 512Mi
stackIcon:
  name: type-che7.svg
  mediaType: image/svg+xml

[skabashnyuk]

This task goes in a bit different direction.
After discussion with @l0rd and @slemeur we identified that we want to go in the path of Che devfile registry. Similar to the plugin registry. Stacks API would be abandoned in Che7 eventually.