Support OAuth flow for private repo factories on Dashboard

[mshaposhnik]

Is your task related to a problem? Please describe.

We are about to merge support of OAuth flow for private repository factories on Che server. In next step that this fucntionality must be supported by dashboard as well.

Describe the solution you'd like

Expected flow is the following:

• User accepts factory from private repository;

• Dashboard making request to resolve factory details using the reposiroty URL which is provided by user;
  (POST /api/factory/resolver)

• Che server recognizes that URL points to private repository and looks for the credentials secret; If no secret present,
  initiates the OAuth login procedure by returning the following response:

  POST https://<che-host>/api/factory/resolver
  
  Response Status: 401
  
  Response body:
   {
   "attributes": {
   "oauth_provider": "<provider>",
   "oauth_version": "<1.0|2.0>"
   "oauth_authentication_url": "https://<che-host>/api/oauth/<1.0>/authenticate?oauth_provider=<provider>&request_method=<GET|POST>&signature_method=<rsa|ssha>"
   },
   "errorCode": 401,
   "message": "SCM Authentication required"
  }
  
  
  

Please note that parameters token and redirect_after_login are not returned and must be set by Dashboard by its discretion.

• Dashboard initates OAuth routine using the resulting URL;

• In case of Successful completion of OAuth procedure, initial POST /api/factory/resolve must be repeated.

• In case of Unsuccessful completion, error message must be shown.

Describe alternatives you've considered

Additional context

• Bitbucket server support in Che's factory flow Create a workspace from a URL to a git repository in Bitbucket using a factory link #18385
• Clarify factory resolution process for private Bitbucket repositories Clarify factory resolution process for private Bitbucket repositories #18389

[mshaposhnik]

FYI guys, we're merged responses PR, so the Che server now behaves like it written is description (with configured OAuth).