Theia should accept previewurl links

[sparkoo]

Is your enhancement related to a problem? Please describe.

During implementation of preview url support in the devfile #13945, we've found out that we would need changes also on Theia side.

Describe the solution you'd like

We will send preview url links in response for workspace. The object will look like this:

...
    "links": {
      "self": "http://che-eclipse-che.192.168.42.10.nip.io/api/workspace/workspace7grc58gn0jozwkmz",
      "ide": "http://che-eclipse-che.192.168.42.10.nip.io/admin/nodejs-mongo-ymkmt",
      "environment/statusChannel": "ws://che-eclipse-che.192.168.42.10.nip.io/api/websocket",
      "environment/outputChannel": "ws://che-eclipse-che.192.168.42.10.nip.io/api/websocket",
      "previewurl/runthewebapp_123": "http://ingress111aaa.192.168.42.10.nip.io/hello",
      "previewurl/createtestuser_456": "http://ingress222bbb.192.168.42.10.nip.io/some/path"
    },
...
      "commands": [
        {
          "commandLine": "npm install && npm run dev",
          "name": "run the web app",
          "attributes": {
            "componentAlias": "nodejs",
            "machineName": "nodejs",
            "workingDir": "${CHE_PROJECTS_ROOT}/nodejs-mongodb-app"
          },
          "type": "exec",
          "previewUrl": "${previewurl/runthewebapp_123}"
        },
        {
          "commandLine": "curl -X POST --data '{\"user\": {\"username\": \"test\", \"email\": \"test@test.com\", \"password\": \"password\"}}' -H \"Content-Type: application/json\" http://localhost:3000/api/users\n",
          "name": "create test user",
          "attributes": {
            "componentAlias": "nodejs",
            "machineName": "nodejs"
          },
          "type": "exec",
          "previewUrl": "${previewurl/createtestuser_456}"
        }
      ],
...

Theia should recognize links prefixed with previewurl/ and set the keys as variables with matching values. These variables will be used in command's previewUrl attribute, that Theia should evaluate with these variables.

Describe alternatives you've considered

Send full URL without variables, but this has many issues on Theia side regarding to tasks.json synchronization.

Additional context

see #13945 issue comment #13945 (comment)

[sparkoo]

I've prepared server image with preview url quay.io/mvala/che-server:previewurl. PR is currently in review #14713

cc: @RomanNikitenko