Add previewURL support in Devfile format

Is your enhancement related to a problem? Please describe.

Extend current devfile spec to be allow users to specify a previewURL for a given command

Describe the solution you’d like

 commands:
   - name: run
     previewUrl:
       port: <a port>
       path: <a subpath>
     actions:
       - type: exec
         component: mysql
         command: mvn clean
         workdir: /projects/spring-petclinic

Implementation

• MVP as beta feature. Server side implementation https://github.com/eclipse/che/issues/14893
• Validate devfile with schema with matching apiVersion https://github.com/eclipse/che/issues/14824
• Theia side for previewurl proposed solution https://github.com/eclipse/che/issues/14892
• Theia shows two dialogues when previewUrl is defined in command https://github.com/eclipse/che/issues/14802
• Theia doesn’t update command’s previewUrl on 2nd workspace start https://github.com/eclipse/che/issues/14803
• Document preferences related to preview URL functionality https://github.com/eclipse/che/issues/14999
• Promote to stable https://github.com/eclipse/che/issues/15412

[metlos]

Since we already have something that almost does the job - dockerimage endpoint (or rather the server in the old workspace config model), I’m struggling to see the real benefit of having a previewUrl placed on a command.

The intention seems to be that when the user invokes the command, Theia will know what URL to open to show the web ui of the application the command has supposedly started. Or at least that’s how I understood it.

But this completely ignores the usecase of invoking the application using the terminal or some other means (like would we be smart enough if previewUrl was placed on a command specifying launch configuration? how would that even work if the referenced launch.json contained more configs?).

IMHO, we’d be much better served with adding endpoint support to other component types and adding the path (and maybe even protocol) to the endpoint configuration. In such arrangement, we’d support all invocation modes.

If we wanted to add the support for automatically opening URL for commands, I think we could do it using a command attribute referencing the endpoint, e.g.:

components:
- name: nodejs
  type: dockerimage
  ...
  endpoints:
  - name: web-app
    port: 4040
    path: /app
commands:
- name: start
  actions:
  - type: exec
    component: nodejs
    previewEndpoint: web-app

WDYT @skabashnyuk @sleshchenko @l0rd ?

(I updated the example to use the previewEndpoint wording @sleshchenko suggested. I like it better than my original opensEndpoint).

[sparkoo]

I’ve created an issue for Theia support of previewurl by proposed solution #14892

cc: @RomanNikitenko @l0rd