Add SSO GitLab provider support

[mateuszklysz]

Summary

Add additional support for SSO GitLab provider in SSO Login Services

Motivation

Since Dex supports GitLab, it would be nice to handle this from the dashboard. Many teams use GitLab and such an option should also appear in Devtron.

Proposal

Currently, OIDC has a hardcoded ID on google and this cannot be edited. The best solution would be to add a separate option with a GitLab icon, as it is solved in the GitHub option.

Configuration example:

config:
  type: gitlab 
  name: GitLab
  id: gitlab
  config:
    clientID: $GITLAB_CLIENT_ID                                                    
    clientSecret: $GITLAB_CLIENT_SECRET                                                              
    baseURL: https://gitlab.com                                                                                                                            
    redirectURI: http://devtron.yourdomain.com/orchestrator/api/dex/callback
    groups:
      - my-group
      - my-another-group